Liverpool's more fun than Manchester... right Beergal?
Then again, it's only 45 min. away by train, so you might consider that as a cool thing to do nearby.
I don't think Manchester's anywhere near as interesting as London, but it's a helluva lot cheaper, there's some good nightlife to be found, and there's the infamous Curry Mile if you're into Indian food.
I think 4-5 days in Dublin is good, but I'd give Manchester 3-4 days if possible, to get a bare minimum feel for the city and its surrounds.