Quite a few of the traditional Fife tunes used in the Martial Field Musics (Fife & Drum Corps)of the US Military prior to and throughout the American Civil War (1861 - 5)had their origins in Irish Folk / fiddle Tunes of the period.

A few examples in the repitoire of the 3rd Maine Volunteer Infantry
Co. "A", 3rd MAINE VOLUNTEERS
Regimental Field Music (Reenacting)
Third Maine REGIMENTAL FIELD MUSIC

include:

Minstrel Boy

GarryOwen (Favorite of General Custer of the 7th US Cavalry, and unofficial theme song of the US Cavalry to this day)

My Lodging's on the Cold, Cold, Ground (Melody used for "Endearing Young Charms"). That was part of the US Army's official "Retreat" or "Tattoo" cerimony during and probably before the ACW.

"Low-Backed Riding / Irish Jaunting Car" - Tune later applied to "Bonny blue Flag", previously mentioned.

Remember, if you will, that melodies and lyrics didn't really marry one another all that often back then, and the better ones tended to have lots of intriguing affairs.

"Paddy On A Handcar"; a catchy little ditty. Has anyone ever seen lyrics to that one?

"College Hornpipe", better known as "Sailor's HP". I think that's Irish(?)

"Wearing of the Green", of course, as well as some others I can't specifically recal at the moment, and can't seem to locate my reprint of the 1863 Field Music Manual by Bruce and Emmitt (the same Dan Emmitt of Minstrel Fame).

"Danny Boy". Is that Scots or Irish; I forget... of course there was probably some musical / cultural overlap between the two. We do a somewhat "jazzed up" arrangement to a marching cadence, as well as a traditional slow air for Tattoo.

Several other Traditional Fife Tunes of Irish origin may be found by a perusal of the "Company Of Fifers And Drummer" Collection @:

Fife & Drum Music OnLine

And for all you Fiddlers out there; most Fife Music is written in one or two sharps ("D" or "G") which we understand is familiar to Fiddlers as well as Fifers.

As far as "Civilian" popular music goes, don't forget that "The Last Rose Of Summer" was around just before the War and was fairly popular for a while there. If that wasn't written by an Irishman, the early Irish seem to have taken to it fairly well, and it is best, i think, when sung by an Irish Tenor.

Someone mentioned Prof. Andre Crouch's "Kathleen Mavourneen" written in Ireland in the 1840's, about which I think there has been at least one MC thread in the past.
Used in the Movie "Gettysburg" (a snippet thereof) it is a lovely piece when done well...and that is not an easy task.
If you want to sing it credibly, you'd better be able to hit about 3 octaves and do some vocal gymnastics while you're at it.
It's one I like to attempt while taking a shower or driving (alone) just to see if I can. I sing Baritone, so have to transpose it down a notch so as to fit it all in to what range I've got - I think that it was written with a Tenor in mind, though.

I don't suppose that such "Composed" popular pieces really qualify as "Folk Music" per se., but it seems to me that after the passage of a Century or more there may be a certain modicum of cross-over.
What do you think?

By the time of the ACW, the Irish culture was having a significant impact on American art, culture and music - so sorting it out may not always be easy. Perhaps one of the richest gifts the Irish brought to a young and tempestuous America besides their Music, was their PASSION. Civilization just isn't much fun without some passion, now, is it?

It seems that a lot of that raw primal passion was most eloquently expressed in the Songs they brought over with them.