As far as irish organized crime goes, they did't even challenge the itallians for the rackets, they fell in line and started working for them, if the irish would have maintained contact with gangsters from ireland and organanized irish in all cities they would be a force, now is a good time for them to take the rackets back because the itallians are weaker and they don't want problems
Nowadays Irish Mob = Real IRA. Ruthless basterds who'd cut off your head with a shovel (yes, that really did happen once). But unless you're in a Sons of Anarchy universe, they aren't active in the USA.
