Well I dont speak Irish or Scots Gaelic so I know nothing really, but I'll try tostart the ball rolling till some experts come along. I've worked extensively in Western Scotland and Ireland and talked to people about this, and what I've heard is that there are many variations inside Ireland, but a standardised version is taught in the schools. But traditional native speakers preserve the differences, and Donegal Irish was not(and maybe still is not) easily understood by someone who speaks Waterford Irish,for example. And,as you would expect, Donegal Irish is quite close to Western Isles Scots Gaelic, but still substantially different. It is worth remebering that Scots gaelic was referred to as "Irish" in Lowland Scotland in quite recent historical times.