I have recently had plans for an extension drawn up by Victor Leyden, from Hasset Leyden in Ennis.
Haven't gotten as far as getting a builder yet, not in any hurry with it yet, but I have been recommended Pyne Construction. Not sure if an extension, ok depending on size, would be a big enough job for him, but worth trying.
I would be very interested to hear if/when you do find a builder, who you get.