Scotch-Irish is still very common today (in my part of the world, the Missouri Ozarks, for instance), although the preferred version is Scots-Irish. But, to be fair, one of the foremost scholarly studies on this subject by James Leyburn is entitled "The Scotch-Irish".

I prefer "Scots-Irish" or even "Ulster-Scot", since the latter does a better job in describing the Scots who moved to Ulster (Northern Ireland) before immigrating to the US, Canada, etc.

Cheers,

Todd