Having just returned from there following the festive period, I suggest that you have a banking app or card with Euros on it for the Dublin Port Tunnel. There's also an easy to miss Toll on the M50...
The M50 Toll Road | eFlow
If taking a late boat (and you have not got a cabin) from Hollyhead or Dublin Port bring a blanket and maybe a pillow from the car to the communal areas to try and get a couple of hours of kip. Otherwise it's cold and uncomfortable. And, No, I wasn't on the top deck.
Generally, to use an Irish expression, it is feckin' expensive. But I'm sure you'll have a great time.