There are no liquid limits to what you can carry on, the 100ml container restriction is just an airplane thing.
Bring back as much alcohol as you like and if you are not making it too obvious you will be able to drink it on the 4 hour ferry crossing.
There is also a 2 hour hour fast ferry but that gets cancelled if there is any sort of wind blowing.
Holyhead town is an awful dump, but Holy island is lovely, very like Howth.
That's an awful long walk from Holyhead.
Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch is only down the road
That's another dump all that's in it is an overpriced gift shop, a cafe and a co-op.
Not necessarily. I think it used to be the case that it couldn't dock in Holyhead overnight so if it got to Holyhead then it usually had to make the trip back. I was on an awful crossing on it years ago - my wife and myself were the only people who didn't vomit.
I remember going to that chipper with my aunt in the 80s and I asked her why the chips had so many eyes in them