I can't vouch for these personally but they might work for you:
Broadmeadow Country House & Equestrian Centre
Broadmeadow is an exceptional country house, with a purpose built modern equestrian centre on 100 acres of farmland. With a large indoor and outdoor arena, tennis court and spectacular gardens you can unwind and relax or enjoy an active break. Broadmeadow is located 15 minutes from Dublin Airport and 25 minutes from the city centre. Packages can be tailor made to suit individual/group requirements.
Or
Royal Meath Equestrian Centre is set in 250 acres of beautiful Meath panoramic countryside. Fully approved by AIRE and BHS. Superbly located 19 miles from Dublin city centre and only 15 miles from Dublin Airport (free transfer) - http://www.ehi.ie/facilities.asp?facilityID=7
Be a bit sceptical of transfer times - traffic in Dublin is pretty bad.
The better equestrian centres are located outside of Dublin. Kells is one of the best. Just be a bit careful of hacking in mountains - did go on one of these in the past and it was extremely dodgy!!
