Kilmainham jail is one of my favourite places to bring people if you're looking for a bit of history. Also Glasnevin cemetery.
If you want good during the day pints: Grogans and have a cheese toastie while watching the people go by.
And if you're into your fancy cocktails: Vintage Cocktail Club in Temple Bar is one of the few places worth visiting in that tourist trap - I'd recommend a booking at weekends. Also a lovely place to take a girl on a date, she'd be very impressed.