OP, be wary of a lot of these recommendations, some of them are very much a meat market for the older party lover, but you're as likely to find swingers and over 50's as you are an older than the average night club goer venue.
I think nightclubs and the over 30's is a bit of an oxymoron. Mostly because the nightclub scene has gotten very old by that stage. (in my late 20's and have been over night clubs for a few years now), the best place to meet people and have a great night for the slightly older crowd is usually in bars that can have live music etc., but not a night club really.
There are lots of bars around Grafton st that personally I quite like, such as Keoghs, The Duke, other bars like O'Donoghues on Baggot St. (and most Baggot St. bars) have an older crowd, Staggs head, 4 Dame Lane is more nightclubby, but can have an older crowd, also places like Sin e and the Stephens Green end Camden St are decent.
A lot of places in the suburbs too, such as Russels in Ranelagh, which is a good area for the 30 somethings to socialise.
If it's strictly nightclubs you're looking for, then a lot of the over 30's and specific 80's / 90's nights in clubs as suggested are your best bet.
Just a few ideas, hope it helps.