This is fascinating stuff!

In July 1863 Lieutenant J.L. Capston, a cavalry officer, received a letter from Confederate Secretary of State Judah P. Benjamin indicating that he was to be reassigned. His destination was Ireland, and his task was to use legitimate means to counteract the work of agents of the United States operating there. His ultimate mission was to prevent the Irish from emigrating to the north and enlisting in Federal armies.

via Quick Link | A Confederate Agent in Ireland « Irish in the American Civil War.