DV are spot on Dave Vincent is the guy to ask for and you will also find Steve Phillips(TDI UK ) dives there.
They tend to concentrate on Strangford lough.
You have also got Aquaholics on the north coast who go out to Donegal (u-boats) and Rathlin.
Dive north who i have never used but have good website and from what i here are pretty good to.
Mevagah dive are in Donegal to but thats a fair distance from belfast.
I have posted a few links to a couple of sites hope this helps
Scuba Diving Equipment - Dive Club in Ireland - Aquaholics http://www.divenorth.com/ subindex