The Clancy Brothers used a 5 string quite a bit as so did the Corries. Some may argue that these groups are more folk than celtic but... Elderly instruments used to carry a book by Tony Sullivan which used the Seeger style for Irish banjo (Sullivan also has a book for the 4 string.)But Sullivan recomends the 4 string for jigs. I tend to disagree with him in that regard. I use my 5 string (Frailing style) alot for jigs I use G tuning & Double C (mostly Dbl.C for Irish tunes) I Do like the rhythmic qualities of the 5 string but I am not too crazy about what some call the melodic style. The rhythmic qualities are obviously great for dancing