Margo, the plectrum banjo is basically a 5 string bajno minus the 5th string and is rarely used in Irish music. The most popular banjo for this type of music is the tenor banjo which again has 4 strings, is played with a plectrum but has a smaller scale length and fewer frets, typically, 19 or 17.

There was a thread recently that you might like to check out as it lists several great Irish style players amongst other things. It is at http://www.mudcat.org/thread.CFM?threadID=22063.

Jon