So, I was reading about the Brigantes, a Celtic confederation in the north of England. Apparently, this confederation incorporated septs not just from England, but also from the south of Ireland, around Waterford. How would this confederation have been ruled? Was it a hereditary kingdom (I know the Brigantean queen famously betrayed Caratacus to the Romans), or did each sept take turns at ruling? How autonomous would the individual septs have been?
Would all of Brigantia have spoken the same language, or would different septs have unintelligible languages? How close would the language of Brigantia have been to the language of the Carvetii to the north, for instance? Have these languages ever been reconstructed by linguists, or were all traces of the ancient languages obliterated over time (there appears to be very few Celtic loan words in English, no?)
Those in central and eastern England, as well as those in the Scottish Lowlands, mostly spoke some sort of Brythonic language, the forerunners of modern Cornish, Breton, and Welsh. Many in Western England, Eastern Ireland, and the Scottish Highlands spoke Goidelic languages, which were the precursors to modern Irish, Manx, and Scottish Gaelic.
All of the Brythonic and Goidelic languages emerged from a version of Celtic (called Insular Celtic) that broke away from the mainland versions (Continental Celtic).
So, in theory at least, Irish and Manx (Goidelic - Goidelic) are more closely related than Irish and Welsh (Goidelic - Brythonic), but Irish and Welsh are more closely related than Irish and Gaulish (Insular Celtic - Continental Celtic). Irish and Gaulish would be nearer than Irish and German (Celtic - Germanic), while Irish and German would be closer than Irish and Basque (Indo-European - Basque Isolate).
Of course, that leaves out the Picts. Their language was likely Brythonic, but maybe not. For all we really know, Pictish may not have even been an Indo-European language, though it probably was.
I wouldn’t say that the septs had languages unintelligible to each other–they all come from the same base, after all. Plus, while they’re not itty bitty islands, the Isles aren’t that big, so areal influence would play a major role.
I don’t know about the Brigantes, but I am confident that there was no effective rulership structure extending over northern England and southern Ireland. Later there were Celtic “kingdoms” that extended between Ireland and Scotland, but these were separated by a much narrower stretch of water.
About the languages, you are right that there are quite few Celtic loan words in English (though there are quite a few in French, including some - like valet - that have subsequently been borrowed from French to English. valet would be cognate with the word ghillie which was borrowed into English from Scottish).
My guess is that at that time, all speakers of Irish (Goidelic) would have been able to understand each other, and all speakers of British/Welsh (Brythonic) would have been able to understand each other. That guess is not based on evidence, but on the fact that there had been less time for divergence within these languages.
I think the OP is referring to the time of the Roman occupation, at which time the Goidelic languages were restricted to Ireland and the Brythonic languages to Britain. Only later did the Irish export their language to Scotland, and later again to the Highlands.