Wondering how many Irish who fought in the US Civil War had been in the British Army prior to emigration?

I had a great grandfather who was in the British Army when he was scheduled to go fight the Russians in 1854. He embarked for New York instead and immediately joined the U. S. Army. The unit came west and took part in the 'Navajo War.' During the Civil War, he was in the forces that opposed the Texans under Gen. Sibley in New Mexico. The fate of some of Sibley's soldiers is told in the song, "The Santa Fe Volunteer," written at the time. After that action, he transferred to the Commissary department, handling cattle.