Is there a rugby club in Patagonia?

Plenty I'm sure. The nearest town to the Welsh speaking villages is Trelew and apparently it has two: Trelew and Patoruzú rugby clubs.

Do Irish players that retire whilst playing in Ireland not get a tax rebate for all of their playing earnings? I understood that was one of the reasons Sexton returned. I suspect JL, BA and JGP will stay around long enough to collect that.

Professional athletes who spend at least 10 years of their career competing in Ireland can claim a tax rebate of up to 40% of the tax they paid on Irish income from their sport.

You don't need to retire in Ireland, so long as you've completed the 10 years of tax residency.