Well, Matt beat me to it. He is correct. There are a group of MacKays who are a sept of Clan Donald and have no relation to the northern MacKays. A lot of the Northern Ireland MacKays (or names of various spellings; e.g. MacCoy) are from Clan Donald and not from the northern group.

My humble suggestion for anyone with the name MacKay (or variation of the spelling) would be to do your research. If you're certain that your MacKays are from the northern group, I would join the Clan MacKay Society. If you can prove they are from Islay or Kintyre, I would definitely join the Clan Donald Society.