The New York Times has experienced reporters in Britain and Ireland so one assumes this mistake crept in via an editor --
Members of the groups are mostly drawn from the mainstream I.R.A. and its hard-line offshoot, the so-called Provisionals, who were the main republican strike force during the 30 years of sectarian violence known here euphemistically as the Troubles.
The "mainstream" IRA during the Troubles was the Provisional IRA.
Incidentally, one remnant of the once mainstream "official" IRA, Sean Garland, is back in the news. But it's an era that most people have forgotten.