Heavenly Coffee on Temple Bar Square, though you can't sit in there, and I am not sure how long it is open in the evening- I think it may only be 8pm in winter, and 10pm in summer.

Also, Hagen Daz Cafe on Wicklow Street, they are open till 11pm, superb hot chocolates and also some interesting flavours, such as hot chocolate with a taste of coconut etc.