The IRA effective units were small but were part of a large organization with international connections and good internal intel and commo. The larger organization picked where and when and made sure things were set up properly.
Look what a platoon of Marines can do, with the backing of a few tens of thousands of people, driving subs, controlling satellites, collecting intel and info. Without that backing they’d be a dangerous bunch for sure but much easier to deal with.
A few small bands on their own would not be that effective.
Look at a smakll neighborhood gang vs the Crips.
And then there were two guys with one rifle and a ratty old beatermobile who darned near shut-down the DC area for a couple of weeks a few years back, passing through roadblocks with impunity, and, to the best of my recollection, no external support.