I used to live in Dublin and still visit it regularly. I agree that it is drab, dirty and a bit dull. The Guinness storehouse is a huge disappointment - it's got no actual heritage or museum pieces and is mostly about the ads, and the 'free' pint of Guinness.
Phoenix Park may be large, but it's flat and dull and you have to dodge cars as they can drive through it. I never really enjoy my visits there.
The Book of Kells is worth seeing, and the Glasnevin cemetery tour is good.
Yes you can get the dart out to the coast, but it's not a particularly nice coastline - not as nice as many parts of the coast in NI within reach of Belfast.
Dublin has no nice riverside or docks area that can compare to the stylish and scenic Titanic quarter in Belfast, with its views of the Cave Hill.
Overall I think Belfast is prettier, with some nice parts on the banks of the Lagan eg Waterfront, and surrounded by hills and mountains. The Botanic gardens in south Belfast is pretty and very Victorian (with a newly restored tropical ravine to add to the palm house) and the university area has a nice vibe. The Ulster museum is there and well worth a visit.
Just outside of Belfast there is the open air musuem (folk park) and transport museum at Cultra, on the shore of Belfast Lough.
I've never done a black taxi or walking tour of Belfast but people rave about them