Harry McCarthy, born in England (1834) of Irish parents wrote one of the most popular songs of the CW, The Bonnie Blue Flag. Versions of this soong were sung by both sides.

John Thomas, (born in Wales) emmigrated to NY was an opera singer. He wrote THE SOLDIER'S RETURN, a popular song during the CW.

Patrick Sarsfield a native of Ireland, emmigrated to the Boston area and wrote WHEN JOHNNY COMES MARCHING HOME (under his psuedonym, "Louis Lambert").

There's probably lots of scholarly stuff on this subject - but these are few bi8ts of info I knew about ...