To reiterate what everyone said: London over Dublin. Hands down.
Don’t get me wrong. Both places were fun. But I’ve spent 3 days in Dublin and really felt I’d done everything I wanted to do. I’ve spent a week in London and didn’t get to even half the things I wanted to do.
You’re doing it wrong. 
Hmm okay, after thinking about it here is what I would do in Dublin for fun:
Bram Stoker museum as well.
Never been, but heard it is great.
National library gets a vote too.
Tour of the Dail is interesting but hard to get.
Very good itinerary especially the Howth and Dun Laoghaire walks if it’s not raining. Get a DART from the centre of town and for a few quid and less than an hour trip it takes you to this
Oh totally forgot about the seals- Odessa is seconded- yum!
Don’t drive in Dublin, for the love of all that is holy.
Not only is the traffic a bastard and parking scarce and expensive, or streetside and not particularly safe, but the Irish don’t have much of a concept of jaywalking.
If you want to cross the road in Ireland, you look both ways, and if it looks safe enough, or the traffic is slow enough, or you think you can spot a gap, you cross.
We do have pedestrian crossing with lights and signals and all that too, but they’re not at every intersection and if someone thinks they’re too far away/too inconvenient they won’t use them.
So, not only would you be driving a hired car on the wrong side of the road through crazy traffic on very narrow roads, you’d also have mad Irish people walking out in front of you at fairly regular intervals.
Less people die than you would think, but still, it is disconcerting to North American tourists.