My degree is in early medieval archaeology, so I came across a lot of scholarly research on the political organisation of Anglo-Saxon communities, and a lack of such on the British and Gaelic communities of Western Britain. What there is of western Britain seems to be on kingship, but what of law, land management, administration (which we know in an ecclesiastical capacity), church.
We know Anglo-Saxon and British/Gaelic kingship was not dissimilar, my question is therefore, was there a Brythonic or Goidelic version of the witan, the moot or the witenagemot?
Hey, so I answered a question here (specifically relating to women and power in Anglo-Saxon England) that touches on the big problem with historical ideas of "THE Witan/agemot."
The idea that the Witan was some kind of formal institution to which people were elected is largely a Victorian creation designed to provide a teleological justification for British colonialism and cultural superiority under the guise of Britain being the 'home of Democracy', and one that really is past its time. That's not to say that there wasn't any type of Witan at all, but that the idea of it as some sort of permanent legislative institution is really an invention of 19th century Whig historians who wanted to see a reflection of themselves in England's history rather than an accurate representation of actual Anglo-Saxon governance.
"The Witan" really needs to be contextualised as "a Witan" - a council of nobles and worthies who could provide counsel and ratify decisions at any given moment. These can be most readily seen in the attestations of the many and various extant Anglo-Saxon charters; the local ealdormen, reeves, thegns, bishops, ministers and hundredmen who attended the king's itinerant court at any given site were his witan at that specific moment. Women do appear in these lists, but rarely, and the vast majority of these figures appear to have been male. Really, we should see it as an alternative term for the 'court' of any later medieval or early-modern monarch.
In that regard, we can definitely insinuate that contemporary Welsh and Irish polities would have had their own 'Witans', in so far as rulers would have had attendant courts of their nobility. When the king of Powys, Hywel Dda, codified his Æthelstan-inspired legal codex, the Cyfraith Hywel at an assembly at Whitland at 945, for example, that would be a perfect example of a 'Witan' just by any other name.