In the Hebrides a few years back a friend of mine from the southern part of the Irish Republic began speaking Irish to a young Scottish lady who spoke Scottish Gaelic. (We were making a purchase at a bookstore and the young woman was the cashier.)

She laughed with amusement and replied in Scottish Gaelic. The two of them chatted on for several minutes. I asked if they'd had any difficulty in understanding each other. Both of them had learned their second language in school, and both agreed that they had no trouble at all in understanding each other, but that each sometimes used idioms that sounded either old-fashioned or slightly odd to the other, and their accents differed as well.

They both seemed to feel that their respective school systems tended to emphasize the degree of difference bateween the two "Gaelics" rather than their similarities.

My impression was that they had far less difficulty understanding one another than, say, the average American or English urbanite has in comprehending old-fashioned spoken Lowland Scottish English of the Burns variety.

Just an anecdote, but illuminating.