The Scots are in fact originally Irish, migrated from Ireland. Then came the plantations when many Scots came back to Ireland and settled where they were given lands stolen from the Catholic Irish. The reason NI has Presbyterianism is because of the Scots Presbyterians.
Particularly from Mayo and Donegal the "tattie hokers" were seasonal migrants to Scotland every year when the potatoes were lifted and many carried on down to Lincolnshire to lift the beet there. There is a history of disaster with regard to the tattie hokers, the Kirkintilloch Tragedy, and you can read more and hear the instrumental of the song I wrote about it here.
