Perhaps centrally located Markievicz Pool, Townsend Street, Dublin 2; a deck level pool in the city centre on the south side about a 5 minute walk from O'Connell Stree Bridge. Don't carry valuables around that area especially after dark.
Unfortunately the're building swimming pools alot more in Dublin without proper deep ends probably to save water (& water heating & treatment costs) & lifeguard wages.
The Aquatic Centre way out in the suburbs near Blanchardstown is as the name implies. I've never swum there.
Some of the colleges probably do too and they rent time out to clubs though I doubt you could just go in off the street & use them. about 7 years ago
You could also try the ESB (Electricity Supply Board), not that centrally locatted in Ringsend on the southside, near the mouth of the Liffey. Not sure if it's open to individuals though and the public transport out that way is pretty dire. http://www.sportsco.ie/facilities/swimming-pool/esb_swimming_club.html
In Stillorgan you could try Glenalbyn way out in the southside suburbs. http://www.glenalbyn.ie/swimming_pool.htm.
Also Alsaa near Dublin Airport on the northside.. http://www.alsaa.ie/
Private clubs http://www.energiefitness.ie/about.htm Have a nice deck level pool in Rathfarnham. Need to check pool depths yourself in all pools to ascertain if they are to your satisfaction.
Private school pools:
City Centre: Belvedere College,
Ballsbridge: Marion College. about 7 years ago