"the Kerry and Donegal traditions, for example, are wildly different from each other"
This is true - but only to an extent Malcolm.
The two styles you mentioned are certainly still to be found, but have been greatly eroded; many other regional styles have disappeared completely.
The influence of the Coleman era in introducing Irish American records severely dented regionalism, as did the emigrations, where musicians brought back the very much neutralised styles they picked up from playing with others from different parts of Ireland. The tendency of CCE teachers to teach for competitions has produced what is often referred to a 'a Comhaltas style' of playing.
There has been a series of TV programmes on regionalism recently, the constant theme of which was its disappearance. It was suggested that in many places regional music was identified as belonging to a specific area by repertoire rather than style.
Jim Carroll