We enjoyed our time in Belfast when we toured Northern Ireland. We ended up in Enniskerry after overnighting in Letterkenny, so I don't know how long the trip is to Belfast. But we spent 2 nights there and enjoyed walking around, saw some live music, and took a black cab tour of the murals, etc.

Another tinfoil hat wearer joins to post on his rabid cause of the day.

To the OP. It would be quite a way to go for just a one day trip but if you could stretch to an overnight there's lots to see and do.

Why do you ask?

There's next to nothing in Belfast worth making a specific journey for most people. If you've got a genealogical interest, a fascination with the history of the Troubles or want to see Britain's best-preserved working-class industrial houses, you might think it's worth the 5-6 hours of driving getting there and back will involve.

If you've GOT to be in Belfast for some reason, there are a few things to pass your time pleasantly (like the preposterous Titanic museum that trades on Belfast's inability to build a safe ship, though doubtless someone's managed to blame the English for that. Everything's always someone else's fault in Belfast), and you'll likely enjoy yourself as much as anywhere else in Ireland.

But the really nice countryside is beyond Belfast and pretty well unreachable in a day for most people. It's a pretty boring drive to Belfast, and if you're thinking of getting the train from Dublin - very, very boring.

DON'T visit Belfast to watch the parades during the marching season (early July) unless you know exactly what you're doing. Away from the marching zones at that time of the year, though, Northern Ireland's a great deal safer than wherever in America you live. Ignore the lunatic claiming otherwise

Our daughter did a semester abroad studying in Belfast, so we visited her there when we went to Ireland/Northern Ireland. While we enjoyed our day in the city, I think the best part of our trip was spent outside of Belfast, particularly along the Antrim Coast.

If you can arrange for 2 days/1 night in Northern Ireland so you can visit both Belfast and the Antrim Coast (Carrick-a-rede Bridge, Giant's Causeway, Dunluce Castle, etc.), then I think the drive from Enniskerry might be worth it (around 3 hours). If not, I would suggest finding something closer to Enniskerry for a day.

Driving is about 2 hours each way - so either a long day trip or an overnighter isn't out of the question. The new Titanic exhibit is open, and I've heard it's pretty cool. However, there are lots of things near Dublin I'd explore first! I do like Longhorn55's suggestion of a couple days along the Antrim coast, which is absolutely stunning.

I went on a day trip in April. It was by train and we picked up a tour at the train station. From the little I saw, I was not very impressed. I very much wanted to see Giant's Causeway which was the only reason we went and it was a long day. I don't know what I was expecting and glad I went but don't need to go again. This of course is only my opinion.

NOT - Given the recent decision by the Belfast City Council to euthanize Lennox, a dog that looked too much like a pit bull I will never travel to that region again. They may decide to murder me because I don't fit their norm for how people should look.

I love visiting capital cities, so Belfast was essential when we were in Dublin.
@Two hours train each way and we spent the day touring and walking the city.
We happened to be there the day the Queen Mother died.