I’ve always thought that the Celtic tribes in England gradually intermarried with the Anglo-Saxon and Norse settlers, and took on their customs rather than there being any sort of mass wipeout of people ala the Native Americans. Genetically the English and Scottish are very similar to the Irish and Welsh and if there was a genocide of Celtic people they would have far more affinity to continental Germans and Scandinavian people.
It doesn’t seem like there was always bad blood between Germanic and Celtic people either - their cultures merged pretty gracefully in places like Northumbria where you had people speaking Anglian whilst practicing Celtic Christianity and the Isle of Man, Scottish Isles and Cumbria where you had a robust Norse-Gaelic culture.
People often cite the lack of Celtic place names in regions like Yorkshire as evidence of a genocide but I’d note that Native American names persist in America despite a near complete population replacement, so the fact that the names were changed doesn’t necessarily mean the people themselves disappeared.
Place names are one of the things that change the least. Consider the many places in the US that have been under multiple regimes without a major name change. E.g. Los Angeles was originally a Spanish city (El Pueblo de Nuestra Señora la Reina de los Ángeles del Río de Porciúncula, i.e. The Town of Our Lady the Queen of the Angels of the Porciuncula River). When Mexico gained independence from Spain, it took LA with it. Later on, the US took much of northern Mexico and filled it with English speakers. People still call it Los Angeles, not “The Angels”, or “New Americatown”.
There are a couple of historical events that came to my mind when I read this. Neither of them are exactly a genocide of Celts, but they were major upheavals.
The first is the Harrying of the North by William the Conquerer in the winter of 1069-1070. Admittedly, the target wasn’t Celts, but the north of England near the Scottish border. Still, it appears to have been a real genocide, and created a virtual desert between Norman England and Scotland. The Domesday Book shows it in pretty stark terms; estate after estate in the north of England simply has nothing on it.
The second is the Highland clearances after the battle of Culloden/Drumossie in 1745. While the battle itself wasn’t a genocide, many people starved or emigrated in the aftermath and significant parts of Celtic culture were suppressed.
Like I said, neither of them are directly applicable to your question, but it’s all I’ve got.
It may not have been active genocide so much as disease. Around the middle of the 6th century,the Plague of Justinian supposedly hit the native Celtic (i.e., Romano-British) population harder than the Germanic (i.e., Anglo-Saxon-Jute) settlers. With the country largely depopulated, the Anglo-Saxons moved into the rest of what is now England and were able to easily impose their language, religion, and culture.
Then why are English people still so closely related to these nations, as well as to the Irish? I suppose maybe Celtic and Germanic people are just fairly close to begin with, perhaps both being “Nordic” northern Europeans.
The Anglo-Saxon invasion ( if invasion is even an appropriate descriptor - it is still argued ), drove Celtic polities to the edges of Britain. It’s not likely to have significantly altered the underlying strata of the population of England as a whole.
Thus is the case with virtually all foreign invasions. The more modern historians take a closer look at conquests throughout history, the more we see that previous assumptions about wholesale replacements of one people with another probably never happened or did so only very locally. Modern Egyptians don’t speak Arabic because the Arabs exterminated the native Egyptians and replaced them - very few Arabs actually settled in Egypt after it was conquered ( and most of those stuck to new garrison towns ). They speak Arabic because Roman-era Egyptians adopted the new universal lingua franca of their conquerors.
It’s exceptions that stand out. One of the remarkable things about William I’s conquest of England is how virtually the entire native ruling class was disenfranchised and replaced by mostly French-speaking continentals in just a single generation. Even then the peasant population remained intact ( well, when not harryed to death by endemic warfare ). In the Americas you has massive die-off from disease. But these stand out as a fairly unusual events.
Can I nitpick - the bulk of the Highland Clearances went on for many years and had nothing to do with Culloden or anti-Celtic activity. They were essentially an internal class war where the Scottish clan chiefs booted their own peasants off the land in order to establish huge estates of their own (mostly for sheep farming.)
I’ve been reading Leslie Alcock’s Arthur’s Britain during lunch and laughed out loud when, having gone through the steps a place name apparently went through from Old Welsh to Saxon to Old English to Middle English, he commented that the roots of the modern English people’s inability to pronounce foreign words correctly goes back a long way. It doesn’t help that, as near as I can tell, whoever first wrote the Celtic languages down had only the vaguest idea how letters are used or how they sound. Much like when St Cyril created the Russian alphabet while trying to keep straight in his head the Roman and Greek alphabets he had learned as a child but hadn’t used since. That’s my theory and I’m sticking to it!
BTW, that’s a great book! First published in 1971, when he revised it slightly in 1987 he said that more-recent studies have shown that much of it is rubbish. But Penguin republished it anyway, and appears to do so still. The first 120 pages or so are an attempt to build a chronology of Dark Age Britain out of too few and too contradictory references–many lunches (I read slowly) of edge of my seat thrills–followed by trying to work the absolute minimum of archaeological evidence (not much survives in a wet climate, and many techniques were in their infancy; much of what I learned in archaeology school at about the same time is wrong or can only be used in dry climates) AND those worthless documents into a useful history. Plenty of “Bede says this, Gildas says that, and this Easter annal says another thing, but most of it was hearsay and it’s sometimes impossible to know what calendar they used, so this event could have happened any time between 450 and 575. Or in 363, if it even happened.”
Alcock, so far, thinks the genocide was overstated because there is too little archaeological evidence, like charred remains of buildings and corpses in the wrong places. He cites one site that was long thought to be a site of a massacre, but further study showed to be a plain ol’ graveyard. I think he’s going toward “some of the locals got shoved along, some died in one of the many plagues, but most stuck around and married the new folk.” But at this rate I may never finish the book.
There still seems an element of cultural genocide there. Seems to me the English have long gotten their way in their ‘possessions’ by rewarding the nobility and telling them what they should do in order to obtain more rewards. Too bad they didn’t think to use that strategy on their own descendants in the Americas; all that unpleasantness during the 40 year perior starting with 1773 might have been avoided.
I hear the Scots don’t raise sheep now, and you can’t buy decent woolen goods there anymore.
Ireland is not in England, as per the OPs question (and nor is Scotland, for Sattua’s benefit).
As Tamerlane points out, the idea that all the Celts upped sticks and moved to Wales, Cornwall, Ireland and Scotland is really a misinterpretation of the dominance of Anglo-Saxon culture in England. The Celts were still here, they just adopted the language and religion of their conquerors. I imagine most current English people are a healthy mixed breed.
That’s what I always have read as well- the Celtic rulers and culture got pushed into Wales, Scotland and Cornwall, but the actual peasants who worked their farms more or less stayed put with new neighbors and new overlords. It was similar to the Viking invasions and the later Norman Conquest- the overlords changed in both cases, but the people stayed the same- nobody forced the peasants off the land.