If you have never been to Ireland before, Dublin is a must. Plenty to do if you have a day to spare, plus very accessible by air from anywhere in the UK. The usual tourist stuff is pretty good actually - Guinness Brewery, Kilmainham Gaol, Glasnevin Cemetery, plenty of nice museums and of course the Temple Bar area which is the hub of music, nightlife and 'craic' in the city. If you plan to spend a little longer than one day, I would recommend an open-top bus sightseeing tour on the first day (weather permitting) and thereafter making a note of your favourite sights to visit later/the next day.