The Cois Fhairrge dialect is a Connacht variety but was influenced by the Munster dialect of Co. Clare. This is evident in the Aran Islands. See Ó Siadhail.
Is there not a standard Irish that wee poor galoots could learn? Of course there are local varieties of every language but I struggle with this course. I always used to understand Africans speaking French better than French people because they spoke French by the book. Later I learned a more Normand speech with student jargon but BASICS FIRST.
I believe it was the Danes/Norsemen, but I don't know whether native speakers would understand Gall as foreigners or specifically as Danes in this case, hence my clumsy attempt to hedge my bets.
For gall, Dineen offers "a foreigner; applied in succession to Gauls, Franks, Danes, Normans and English," which I think is an improvement over the FGB's rather dry definition. It also ties in nicely with your "various sets of intruders."