Guinness is abundant wherever you choose.
As WillK says Dun Laoghaire has fuel on the water (ladder climb so if you’re lazy refuel near high tide) and a large marina so they always have space.
DART is urban rail along the coast so will get you into city centre.
Howth is a fishing/seaside town just North of Dublin Bay, also has marina but no petrol on the water. Good choice of restaurants and pubs. Again you can go into city on the DART from there too.
Malahide also has marina, I think they still do petrol. Bars and restaurants here too. The ubiquitous DART also runs from there. Be careful in a strong easterly as there’s a bar and narrow, but very manageable, channel.
If you’re doing a tour, Greystones is just South of Dublin with a marina and some good food outlets. And yes, you can get the train into city centre from here too.
Dublin Port like you to talk to them once inside Dublin Bay buoy. Particularly if you want to do a run up the Liffey.
https://www.dublinport.ie/water-safety/
There is a small marina up the Liffey at Poolbeg but don’t think you’ll get fuel and no train from there.
Enjoy the trip.