Well most Irish crooks have achieved a level of legitimacy over the century. As the most populous immigrant population at the infancy of America, they became slowly entrenched in the political machine, in the police departments, fire departments, other official position as well as on the streets and in the business world.

As far as the things that made the Mafia so properous in decades past was it's insulate nature. Sicilians (whom birthed the American Mafia) have what I term an "invader culture" which opted them to be more secretive, clanninsh, and distrusting. WHich is why certain crime families were formed the way they were. The Irish didn't have that degree of inter-ethnic segregation, altho they did have that mentaility from being lorded over by the English for centuries*

In the end, Irish OC became more locale based. Bosses arising spontaneously in Irish ghettos. The Hells Kitchens, Southies, Northeast Phillys. And to the contrary, LCN families were more compact and almost fuedal. How bosses would arrange intermarriage to other gangsters. Something very unique amongst Italians.

And for this reason is why Italian never really entrenched American society the way the Irish did. Using organized crime as a tool to get ahead in the world. So to answer your question. As long as there's IRish neighborhoods, there will be Irish organized crime. Without them it falls onto white collar and political corruption. However the Italians, have a structure which has outlasted that. But it quickly dying due to Americanization and degradation. Rich gangster having spoiled kids for example. And ass too do the Italian neighborhoods die, this also hurts the families as recruitment suffers. But there's also the fresh blood coming in through intermarriage and sons following in their father's footsteps. It's really a question of organization which the Irish have been lacking, but made up for in ambition.