No, not slang for the cops. Organized crime, Irish-style.
Like The Mafia and The Mafiya the Irish Mob is strongly linked with poor immigrant communities. Mostly this means America but they can also pop up in stories set in Britain as well as back home.
A distinction should be made between the Irish Mob and groups like the IRA and the INLA. While there is some overlap in fiction (and reality), for [...] [tvtropes.org]