Kinahan cartel seem to be the most powerful irish organized crime for now.
There alliances with other groups, and there presence in manu countries in Europe seem to be so powerful.

After, I would go with the West end gang. During the 70s, the FBI put the leader of the west end gang in the top 5 biggest drug dealer in the world (or the continent)

I guest the third spot would be between the winter hill gang and the westies.