I'm going to be in Dublin (with a friend) for 3 nights and 2.5 days coming up. Should we take a day and go up to Belfast or just do Dublin for that time period?
If you can spare the time you really should see Belfast, it's a beautiful city with a really warm-hearted welcome.
I have always said that Edinburgh and Belfast are the two most under-rated cities in Europe.
Belfast is really great, but if it is your first trip to Dublin and all you have is 2 1/2 days, I'd say you'll have more than enough to see right in Dublin.
You will not regret BFS Belfast. If you have time, include the Antrim Coast. The Giant's Causeway is a spectacular Photog Paradise!
I really enjoy BFS and go there every chance I get! It's interesting to see both Ireland and Northern Ireland, especially during the same trip.
Be sure to start your tour at the Belfast TI downtown. They are terrific travel planners. They've never steered me wrong. You will apprecite their assistance since you will have only a short time there. They are expert at putting together "Speed Travel Highlights Only" itineraries.
Ideally, both cities would require more time if it was possible. You might have to plan Belfast for another time/trip if you really want to get the most out of DUB Dublin.
With only 3 nights, stay in Dublin