"It wasn't till I got involved with the revised Clan MacIntyre history that I learned that the Irish colonized Argyll and the Western Isles in the second century AD."
In fact Argyll takes its name from the Airgialla.They moved into mid-Ulster, probably mid-C4th AD.Together with the movement of the Ui Neill into the NW this had the effect of pushing the Ulaid (from whom Ulster takes its name, of course) into the east so that although they appear to have originally dominated the whole of Ulster they were now more or less confined to the counties of Antrim & Down.
"On a clear day, Antrim in Ireland is within sight of Kintyre in Scotland. Scottish records show a pattern of travel and settlement back and forth between Antrim and Kintyre, with families in Antrim one generation, in Kintyre the next, then back to Antrim, and so on"
And not only Kintyre.The eastern part of Antrim is a plataeau and can be seen across the narrower parts of the channel around Stranraer.Likewise Scotland can be seen from Antrim.Nobody living on either coast would ever have been unaware of the other country.Much of the Antrim coast consists of cliffs.Nevertheless there are a few glaciated valleys which provide harbours and accessible land.Without good roads sea travel would have been easier than land travel.Inhabitants of the Antrim & N Down coasts and their counterparts in Scotland would have been closer neighbours to each other than to communities in the interior of their own countries.