Green 19 is lovely, but the chances of such a large group getting a table are not good - they only take 50% bookings as well, they leave 50% of the tables free for walk-ins. I've gone several times straight after work with my husband and there was no chance of a table (or we could have one for about 45 mins which would feel too rushed).
I'd second the trocadero, also like Salamanca (you can book if you have a table of 6+ I think) - Il Posto does a lovely early bird as well and it's close to the Gaiety.
Odessa has the new Fivers menu which is nice enough - bigger than tapas, smaller than starters. We went and enjoyed it but it wasn't a very leisurely dining experience as they bring everything out as it's cooked so you tend to get through it all quite quickly. Still might be better with a larger group, and you can book Odessa.
Or Harry's on Dawson Street - last time we went they were doing dinner for two incl. bottle of wine for €55. Very popular the place was packed...
M