There's a thread about this on boards.ie. There are two posts on the page I've linked to, which list various options.
Bear in mind that some/most of them are take-aways, though, so not all will be suitable for 'a nice meal/evening out'.
There are also two Moroccan restaurants in Dublin, Dada on South WIlliam Street, and El Bahia on Clarendon Street. It doesn't specify if they'e halal or not on their websites, but it might be worth calling to ask. I've heard good things about the food in both.
Just a followup on this one. I phoned Darwin's on Aungier Street. They were incredibly helpful and ordered in a halal steak for our guest, charged at the usual price for a similar steak.