OP, not sure whether any of these posts address underworld involvement per se, but it may be worth a look through these posts on involvement by Americans in general:

• How prevalent was American funding of the Irish Republican Army?

• United States involvement in the Irish War of Independence/Irish Civil War?

• Did the America government, or it's people, ever support the IRA in any way?

All of these posts are archived, so if you have follow-up questions for any of these users, ask them here, and mention their username so they'll be notified.