I keep hearing that ‘Mc’ is Irish and ‘Mac’ is Scottish

Full-on fiction. “MacIain”, “McIain”, “M’Iain”, “MacKeen” or any other way to spell those two syllables all mean “Son of John” and are the same family – the MacDonalds of Glencoe.

How it’s spelt depends on the fashion of the period and the mood and spelling ability of whoever first wrote the name down.

Travel between Ireland and Scotland was very casual before the invention of borders and passports. That and the practice of using Northern Ireland as a dumping ground for recalcitrant Scots (‘MaHarg’ is or was a common name in Northern Ireland. That’s “Graham’ spelt backwards.)

There are Irish County Tartans available.