When I was a kid, an "Irish" banjo was a four-string, often tuned an octave down from a mandolin, and flatpicking the melody far more often that it played chords. But a few Irish players took to the 5-string, so you would occasionally hear that, though NOT Bluegrass styles, per se. Tommy Makem, for example.
A local "Celtic" band (Dayton Ohio) has a guitar player who doubles on 6-string banjo, playing traditional (mostly-melody) Irish banjo parts, so if I just heard their CDs, I wouldn't necessarily know the difference. (The range is similar.)
I think that when Ben David Warner joined Scythian, he went back and forth between 4- and 5-string. Now he seems to do mostly 5-string, alternating between picking out a traditional melody and some folk-style picking.
So when I tell people the "difference" between Irish and Folk banjo, the lines seem to get pretty blurred in a hurry. That said, Scruggs/Bluegrass style picking is still very rare in bands of Irish or UK origins. The flatted fifths and thirds are pretty much absent, if nothing else.
Your mileage will vary.