Hi,

Bagatelle were never really a "Celtic" rock group (in the same vain as Horslips) but if you mean "Irish" Rock band then yes...sort of. The member you are thinking of is Wally McConville who was the band's drummer. In fact, in the 1980 Hotpress yearbook, the band also lists a fifth member, Margaret Coady as a vocalist (however, she was gone as of the 1981 yearbook)!
I hope that helps you out.. A check of Google shows Wally McConville listed as the contact person for a Irish band called "Baron Quila" these days....although I'm not 100% sure if it is the same Wally.