Any of my friends that want a night or two of luxury in Dublin stay at the Four Seasons in Ballsbridge. I've been to a wedding a few years ago in Brook Lodge hotel and spa in Co. Wicklow, also fab but outside of Dublin.
I would think that Brooklodge has gone down in quality over the past few years so if you're thinking of staying outside of Dublin (as in Wicklow) I'd recommend for pure WOW factor - the Ritz Carlton in Powerscourt. Either there or Druids Glen. Both modern hotels with modern facilities and are really lovely hotels. If you're planning on staying in Dublin, I'd recommend either The Four Seasons hotel in Ballsbridge (near the Aviva stadium/Landsdown Road) or the Merrion Hotel (nearer to the city centre) any of these: The Gibson Hotel (beside the O2 Arena/Point Depot) The Westbury Hotel - just off Grafton Street (nice shopping area) The Westin Hotel - very near to O'Connell Bridge, Trinity college & the centre of Dublin