The 26-year-old came through the Scarlets academy ranks before playing in France with Nevers.
Director of Rugby, Lyn Jones commented: "I would like to thank Carmarthen for loaning him to us for a month.
"Rhys is a talented player who has always stood out as one of the best players in the Premiership- he deserves a chance in Regional rugby."
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland