Roger, you'll find ample information to this question in an interview with Andy Irvine , on the Irish Bouzouki Homepage , or in a history of the group Sweeney's Men .
Here's the most relevant bit from the last page:
"Certainly the most famous contribution of Sweeney's Men (besides some fine music!) was the introduction of the bouzouki in 1967 by Moynihan. This original bouzouki was a 6-string version, which has since been replaced in most groups by the 8-string variant that is now so common. (This variant was probably introduced by John Bailey, who made a flat-backed version with the 8-pegs.) "
Sorry, if this looks like an overkill.