Dublin has 3 marinas Dun Laoghaire, Howth and Malahide (plus Greystones further South and Poolbeg in the Liffey but a little out of town)
Malahide needs a little care around low tide and fresher Easterlies.
Petrol in Malahide and Dun Laoghaire.
All 3 are very easy walks into their towns (max 5 mins)
Howth and Malahide are nice coastal towns with a choice of eateries and a few pubs.
Dub Laoghaire is on a bigger scale, both marina and the town feels more urban. It has a fuel berth and day visitor rate was €5 in 2016.
All three are nice. I've kept my rib in Malahide for last couple of years with no problems, Howth is my favourite and Dun Laoghaire is probably the best from facilities point of view.
If you plan to motor up the Liffey read this
A guide to port entry - Dublin Port
I wouldn't bother though. No formal/secure berthing but you can tie up and send someone up for coffee.
All three marinas are 5 mins walk to train station that runs through city centre.
I haven't crossed Irish Sea by rib so no use to you there I'm afraid.
Hope this is useful.
Enjoy your trip.