That will be a great cruise!
Honestly, to advertise the cruise as going to Paris is a little misleading. Most people we have heard from who have tried to take the 3+ hour drive into Paris, see the sights in a flash, and take the drive back, have been very disappointed. Trying to DIY would be even more pressed.
In Dublin, taking the ship's shuttle dropped us right off at Trinity College to see the famous Book of Kells. It was an interesting area to wander afterwards.
In most of the other ports we took excursions, either ship's or private. Liverpool is a big industrial city and not much to do there unless you are on a tour. So is Glasgow.
Edinburgh is a big place with lots of tourist attractions. Just walking around town would be sort of a waste here, IMAO. I seriously suggest at least a tour of the castle.
We had a great time in the British isles. Check out our trip report and pictures at Royal Princess British Isles Ports