Jim, I just thought to check the introduction to "Learning Irish" by Mícheál Ó Siadhail. He says the following:

The form of Celtic that was to become Irish was brought to Ireland by the invading Gaels -- about 300 B.C. according to some scholars. Later it spread to Scotland and the Isle of Man. Scottish Gaelic and Manx gradually separated from Irish (and, more slowly, from each other), and they can be thought of as distinct languages from the seventeenth century onwards. The term "Gaelic" may be used to denote all three.

All the best,

Mick