Etto on Merrion Row is lovely. Informal but top notch Italian grub (not fucking pasta or pizza, mind)
One Pico is good, but I prefer it for lunch.
Greenhouse as Fagan said is very very good.
Restaurant41 in Residence on Stephens Green is very nice for a meal as well. They have use of the 'club' after for drinks if they want (to be wanky)
Forrest Green would be ok too, or their new place Forrest & Marcy, but that's more small sharing plates. Trendy though if you want to show off your knowledge of hip Dublin
Bastible on the SCR is supposed to be really lovely too but I can't recommend as I've not been there yet
All in I'd go for Greenhouse (swanky-ish) or Etto for great, informal food