Whilst no expert, I do speak Irish whenever possible. My father was anative speaker from the Mayo Gaeltacht.
The Irish took their language with them when they started settling in scotland in the 6th century. Since then, it has drifted away from its parent considerably. The pronunciation is quite different and some of the spellings and vocab is quite distinct from Irish. (made even more fun since Irish spelling was reformed about 50 years ago). There are indeed a number of dialects of modern Irish - plus "book Irish" a standardised version culled from most dialects and taught in schools.
I believe that although Scots gaelic (Gadhilge) has many characteristics of Donegal/Ulster Irish (Gaeilge) it also shares some with Munster Irish.
I originally learnt Connacht Irish as a boy but in recent years have taken up Donegal Irish, spending most summers at a language school in the Donegal Gaeltacht. I find that I can often read Scots Gaelic - even though the spelling looks strange to me, and there vocabulary problems.
(Gaeilge:I am = Tá mé (pronounced taw-may)
Gadhilge:I am = Thá mi (pronounced haa-mee)
I wonder if the relationship between the two languages nowadays is similar to that between , say, Dutch and Afrikaans, or Spanish and Portuguese?
Dáithí