# The gift of the gab: did an iron age brain drain bring Celtic to Ireland?

Saint Patrick, whose feast day is celebrated on 17 March, left behind two short works in Latin, but he probably spoke a Celtic language. By the time he was saving Irish souls in the fifth century AD, linguists are pretty sure that Celtic was spoken throughout Britain and Ireland. When Celtic first arrived in those islands, however, is an enduring mystery – one that new findings in archaeology and genetics might help to solve.

The dates range between the early bronze age, around 2500BC, when archaeologists detect a major cultural transformation in Britain and Ireland, and the first century AD. Most linguists place it in between, in the last millennium BC – the iron age – but they can’t prove it because the language wasn’t written down until much later.

Linguist David Stifter of Maynooth University in Ireland says that an iron age introduction is most likely because the Celtic languages of Britain, Ireland, and what is now France look strikingly similar when they are first captured in writing, suggesting that the three varieties hadn’t parted ways long before. “If Celts or pre-Celts were in Ireland in the early bronze age, we would expect the language to look very, very different,” he says.

The cultural upheaval of the early bronze age may well have seen the introduction of a new language, Stifter thinks, but it wasn’t Celtic; Celtic eventually replaced that language.

Migration is considered to have been a major driver of language change in prehistory, and in 2022 a group of researchers at Harvard University, led by computational biologist Nick Patterson,  that they had detected a big turnover in the British gene pool around 1200BC. From that date until the Roman invasion of AD43 they were unable to detect any other large shift, so they suggested that this marked the wave of immigration that brought Celtic to Britain. “My gut feeling is it’s still a little bit early,” Stifter said at the time.

However, geneticist Lara Cassidy of Trinity College Dublin says that a finer resolution is needed to detect people moving around the north-western corner of Europe in the last millennium BC, because the populations of France and Britain were already quite interconnected by then, and genetically more similar than they had been in the bronze age.

In January, Cassidy’s group  finding identical sequences of DNA in the genomes of people who died either side of the Channel in that crucial period, indicating that they were more or less distantly related – up to nine generations’ separation. “We found evidence of substantial contact between at least the south coast of Britain and France during the iron age,” she says.

The arrival of Celtic in Ireland is even harder to detect, because there’s no evidence of any major reconfiguration of the Irish gene pool after the early bronze age. Again, that doesn’t rule out immigration from genetically similar populations, Cassidy says, and archaeologist Rowan McLaughlin of Maynooth University thinks he has evidence of such an influx.

Human DNA preserved from prehistoric Ireland is scarce, in part because the Irish cremated their dead throughout the iron age. That makes it difficult to estimate how the population size changed over time, but McLaughlin has found a way to do so indirectly. He has built a database of all archaeological traces of human occupation in Ireland starting in the late stone age. At 2500BC, he says, the population was about a million, and it remained that size until 800BC. “We have this big crash at 800BC, and then a few centuries of low levels of activity,” McLaughlin says. “Something big happened, and it’s presumably related to the climate.”





The Roman invasion of Britain, AD43. 

Around 800BC, a global climate event caused western Europe to become very wet. McLaughlin thinks that associated disasters such as crop failures or epidemics triggered an Irish “brain drain” to Britain. This is the date at which the bronze age gave way to the iron age in Europe. Continental societies experienced a major rupture, with new cultural practices and religions replacing old ones, but in Britain it was business as usual – and business was thriving. Population centres were growing, impressive hillforts were being built, and cross-Channel traffic was increasing. “Britain is an unusual place in the iron age,” McLaughlin says.

About 300 years later, the Irish population recovered, which could reflect the descendants of those earlier economic migrants returning to their roots. Only now they may have been speaking a different language, one they had brought back with them from Britain: Celtic.

This scenario strikes many linguists as plausible, but McLaughlin says the smoking gun is missing: clear genetic evidence that Irish people settled in Britain around 800BC. Cassidy’s approach of searching for genealogical links between genetically similar populations could provide it, but she says that it’s not yet possible to rule out a third scenario – one proposed by linguist Peter Schrijver of Utrecht University in the Netherlands.

Schrijver thinks that Celtic could have arrived in Ireland as late as the first century AD, brought by Celtic-speaking Britons fleeing the Roman invasion of AD43. They may have prospered in their new home, forming an elite that managed to impose its language by force or persuasion, or because others wanted to imitate them. An archaic form of Irish could have been what came out of local people’s mouths when they mispronounced what was, to them, a foreign tongue – British Celtic, or Brittonic.

But the split between Brittonic and the so-called Goidelic languages of Scotland and Ireland could have happened differently. Goidelic could have arisen in the west of Britain, before being pushed across the Irish Sea by the Roman invasion. “I think everybody would be very confident that Celtic came from the continent into the islands,” Cassidy says, “but after that, movements could have been in both directions.”

Breton, a Brittonic language still spoken in Brittany, is thought by many to have been planted in France by Cornish people who themselves took to the sea in small boats. Irish slavers seeded Goidelic in Scotland and the Isle of Man in the early middle ages, when they were capturing local people, including the future Saint Patrick.

Does it seem complicated? That’s language for you. If the Celtic story is becoming clearer in some respects, Cassidy says, it’s unlikely ever to be entirely limpid: “The more data we gather, the more questions we raise.”

_Proto: How One Ancient  Went Global_ by Laura Spinney is published on April 24 by HarperCollins (£22)
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