I was listening to the episode of An Saol ó Dheas for January 12th and at the beginning of the episode, there is a guest who begins with what sounds to me like "Go raibh maith agat Helen ar dtúis...". Am I correct in thinking that the phrase "ar dtúis" (presumably the dative?) is what is being used and if so, is this a common Corca Dhuibhne variant of what would be "ar dtús" in other places?

Yes, and yes.

Tús isn't common in Munster Irish other than in the phrase ar dtúis. Tosach is the normal word otherwise. Tús is masculine and you wouldn't expect a dative túis from a masculine word tús, but there are some words that are declined as feminine only in the dative. But the thing is moot anyway, as the word isn't much used apart from this phrase. And in ó thúis (go) deireadh.