According to Jim Hunter at the University of Ulster, the tune we know today as the Londonderry Air was first taken down in 1851 by Jane Ross of Limavady who heard a blind fiddler play it in the street in Limavady. (Today there’s a blue plaque to commemorate the spot). The fiddler was reportedly Jimmy McCurry, a native of Myroe. Here’s Jimmy in the 1901 census:
http://www.census.nationalarchives.ie/pages/1901/Londonderry/Myroe/Myroe_Level/1523675/Though born in Co Derry, Jimmy’s father was from Portnahaven in Islay and was also a fiddle player. Indeed his family had once been bards to the Lords of the Isles who used to rule the southern Hebrides and parts of north Antrim from Finlaggan, on Islay. So Jimmy is bound to have learned some of his repertoire from his Scottish father.
So the tune may be a native Irish tune or, as some have suggested, it may in fact be Scottish. (Not that there’s really a lot of difference). But it amuses me to think that what is surely the most famous Irish song in the world has words written by an Englishman to a tune that’s possibly Scottish.