(Darren) Your example doesn't look grammatical to me and I wonder who put it together. Da means good, but ardderchog means wonderful, excellent etc and you wouldn't use both together. I suspect it should read mae bronnau ardderchog genni hi, literally, "there is (are) breasts wonderful with her". There's no verb "to have" in Welsh. The word genni is a conjugated part of gan, "with", and its ending means you can omit the pronoun it refers to, hi, in this case, and most people would, certainly in conversation.
[Rosie] I've shown it to you before. It comes from an old Welsh-English phrase book, the translation for which it claims is "She has excellent breasts." Quite why they thought that would be a useful phrase I'm not sure.