In general the modern Irish pride themselves of their Celtic descent, but who were the Celts and where did they come from?
As many theories about historical issues, and especially those covering matters which took place before people started to write, there are several ideas about the Celts. Some of these theories are overhauled by technological advances. A relatively new technique in archaeology is for example DNA comparison. Before the discovery of DNA scientists had to rely on the shape of pottery to establish relations between ancient cultures, while modern science is capable to establish family relations over several generations and therewith providing insight in the movements of the actual people instead of the diffusion of technologies.
Modern insights upended the commonly accepted idea of one huge Celtic invasion from Central Europe to Britain and eventually Ireland. Instead it seems more likely that the Celts arrived over a long period of time in a couple of smaller migration waves. Besides solving the mystery why it took so long before Ireland had fully entered the Iron Age, there are many similarities between this idea and the various tribes mentioned in the Lebor Gabal, or Book of Invasions.
Until the second century BCE the Romans and Celts lived comparatively peacefully side by side in continental Europe. Gradually the Roman attitude changed and reached its culmination under the reign of Julius Cesar. During the Gallic Wars in the 50's BCE and by hostile Germanic tribes in north and east Europe the Celts were virtually driven from the European mainland. Nowadays there are seven so-called Celtic countries or areas in Europe: Éire (Ireland), Alba (Scotland), Cymru (Wales), Mannin (Isle of Man), Kernow (Cornwall), Brittany in France and Galicia in Spain.

Éire (Ireland)

Alba (Scotland)

Cymru (Wales)

Mannin (Isle of Man)

Kernow (Cornwall)

Brittany (in France)

Galicia (in Spain)
It might be coincidental, but in most of these Seven Nations are nationalistic movements more or less alive and active. In fact, the Republic of Ireland is the only real Celtic country. Unfortunately the Republic lacks the Six Counties to be a real and complete Celtic country.
The migration to Britain and Ireland is neither the beginning nor the end of the history of the Celts or, scientifically more correct, the Celtic culture. Prior to its arrival on the British Isles the Celtic culture had matured on the European mainland. Nevertheless the exact origin of the Celts is subject of dispute and therewith of defining.
In the absence of a sound yet practicable definition of Celt or Celtic culture it is rather difficult to answer the question who they were and where they came from. The lack of a feasible definition has caused a morbid growth of theories and classifications. Most definitions however agree that the Celtic culture was made up from various tribes which were able to forge iron and shared a common Indo-European language.
The derived term proto-Celts, meaning forerunner of the Celts in its literal sense and most loose definition, is based on the idea of a straightforward development of cultures. According to this idea the Bell Beaker folks, which most likely originated from the Iberian Peninsula and almost certainly brought the Bronze Age to Europe, already were proto-Celts.
In reality however the loopholes, dead ends and gaps attached to the cultural development of the human race can be compared with those in the evolutionary pedigree of our species and the term proto-Celts with this loose definition has become obsolete. Nowadays the term refers to pre-Celtic cultures belonging to the same Indo-European language group as the real Celts.
By this restriction the proto-Celts did not appear around 2500 BCE in current Spain or Portugal, but approximately in 1200 BCE with the rise of the Urnfield culture in Central Europe.
Almost simultaneously with the Urnfield culture the so-called Hallstatt culture appeared. Around 800 BCE the Hallstatt folks got the hang of forging iron, by which they gained the honourable title of First Celts. Nevertheless it took one more step to achieve the culture we commonly identify and recognise as Celtic: the La Tène culture.
There seems to be some sort of consensus that a tribe known as Picti brought iron forging skills to Scotland and the north-east of Ireland in the seventh century BCE. The Scottish Picti became fearsome opponents of the Roman army some centuries later and it was because of them that Hadrian's Wall was built. The origin and nature of these Picti, meaning Painted People, after their painted faces, however is disputable and scientists are not even sure whether or not they were actual a Celtic tribe or just a wandering tribe originating from Northern or Eastern Europe.
The Picti were followed by real and indisputable Celtic tribes from Belgium, France and Spain. These tribes brought subsequently the Hallstatt and La Tène culture to Ireland.
Unfortunately for us this exodus brought the Celts out of reach of the Roman historians and scribes. Consequently we have no idea what exactly took place on Ireland, but it is likely that the sophisticated, highly skilled and artistic Celts suffered a culture shock. They must have been baffled by the megalithic structures and apparently they assumed that they had inherited the land of mythological tribes. The magic, the heroes and the supernatural creatures of these tribes lives on in the tales and fairies of the Celts. In the oral Celtic culture the stories were passed down from generation to generation by druids and bards.
On Ireland, far from Roman influences and the rise of Judaism and Christianity, the Celtic culture went through its first revival. The Celtic culture survived the arrival of Christianity and the Viking era to some extend, but rapidly declined with the increasing influence of the Anglo-Saxons. Increasing cultural awareness in the nineteenth century led to what is known as the Second Celtic Revival.
Although the Celts developed an alphabet, know as Ogham, they totally relied on their oral poetry and the art of tale-telling for their legends. The only written records about the Celts are reports from Roman officers.
The Celtic history slash mythology survived historical torrents by camouflaging themselves as if they were tales about Christian or chivalrous values. Even in modern times the derived legends and their leading figures, such as King Arthur and the magician Merlin and the good outlaw Robin Hood, are well known in the western world.
Unfortunately only a fraction of the ancient Celtic laws survived the Christian and British censorship, but from the remaining volumes of this Breitheamh, or Brehon Law, we can learn that the Celts were not the violent barbarians as described by the Romans and the British. In contrary, the Brehon Law draws a picture of a highly organised, humane and progressive community.
The Irish Celtic society deviated from then contemporary communities in many ways, but especially on aspects such as royalty, succession and landownership.
In stead on just one king, or several kings of equal standing standing, Irish royalty consisted of a pyramid of kings. Each stratum, from the lowest Rí Tuath, loosely translated as King of the Tribe, up to the Ard Rí na hÉireann, or just Ard Rí for short, meaning High King or Supreme King, was granted different powers, rights of property and obligations. Granted indeed, because as a rule the king, regardless its the rank, was elected, so in spite of the extreme hierarchic structure the Celtic society was basically by no means static.
Landownership was a rather complex issue in Celtic Ireland. From a territorial point of view the Rí Cuicidh, or King of the Province, ruled the land the land, whereas the soil and dirt as such was property of the tuath, or tribe, and controlled by the Rí Tiatha, but whatever the land produced, minus a tribute, was generally speaking owned by the féines, or free farmers. If for example the farmer was expelled from his tuath his land passed to the Rí Tuath who in most cases leased it out to céiles, or tenants. By this mechanism the Rí Tuatha eventually became some a large landowner who granted less fortunate farmers to rent not only land, but also cattle or sowing seed and tools.
For the lack of death penalty banishment, and therewith dependency of the benevolence of the Rí Tuath from an other tribe, was the most severe punishment a Celt could face. Luckily most crimes could be settled a fine.
The many intertwined realms, private and tribal landownership and elections thwarted King Henry II when he commenced negotiations with the then Ard Rí Rory O'Connor. The result of these talks, which led to the Treaty of Windsor in 1175, was of little value because the Rory had no authority over Ireland.
Until the second century BCE the Romans and Celts lived comparatively peacefully side by side in continental Europe. Gradually the Roman attitude changed and reached its culmination under the reign of Julius Caesar. During the Gallic Wars in the 50's BCE and by hostile Germanic tribes in north and east Europe the Celts were virtually driven from the European mainland. Nowadays there are seven so-called Celtic countries or areas in Europe: Éire (Ireland), Alba (Scotland), Cymru (Wales), Mannin (Isle of Man), Kernow (Cornwall), Brittany in France and Galicia in Spain.
It might be coincidental, but in most of these Seven Nations are nationalistic movements more or less alive and active. In fact, the Republic of Ireland is the only real Celtic country. Unfortunately the Republic lacks the Six Counties to be a real and complete Celtic country.
Although the Romans definitely were capable enough to launch a successful invasion in Ireland once they had gained a firm foothold on Britain, they seemingly just were not interested. It is a rife misconception though that Romans never set foot on Irish soil. There is evidence of a Roman trading post near modern day Dublin and excavations occasionally produce Roman coins and jewellery. Apparently Celtic Ireland and Roman Britain were partners in trade.
Following the decline of the Roman Empire a German tribe, known as the Saxons, advanced to the west. As the Romans the Saxons showed no interest in Ireland at all and their westward advance came to a stop on Britain. Neglected by the superpowers and remained free from costly armed conflicts Ireland became the wealthiest country in Europe during the Dark and Middle Ages.
For a brief moment the Irish Celts turned to plundering and they occupied parts of western Britain and Scotland. Most of these Irish colonies were taken by the Saxons. All but a small colony in Scotland, named Dal Riata, or Dalriada, which would be the basis for the kingdom of Scotland.
With the arrival of Christianity on Ireland in the fifth century became more connected with European culture. Unfamiliar with the Jewish religion the Irish synthesised the new monotheistic religion with their pagan polytheistic religion (an early form of synergy). By doing so they created a unique version of Christianity where faeries and saints can coexist side by side.