Holyhead to conwy

Conwy is a request stop. You need to tell the conductor when you get on board that you wish to stop. On the way back, you need to raise your arm as the train approaches to clearly signal your intention to board.

As for tickets, there is certainly no manned ticket office. There may be a ticket machine, I can't remember. If not, just board and buy your ticket from the conductor.

Also there is no ticket office and no ticket machine. If you don't already have a ticket purchase from the guard on the train. If this is a day trip the cheapest option is to buy a day return at Holyhead.
That is £19.10

Ok thank you,I’d be afraid the train won’t stop and I’ll miss my boat home lol

Some trains will stop - you need to hold your arm out to indicate to driver.

I have just taken a look at the situation this afternoon - which you can see on this link. The trains marked in faint type will pass through - not stop. Those in bold type will stop but you may need to inform the guard when outbound and hold your arm out when train approaches for return. The timetable will also tell you which trains will call at Conwy. (The Avanti trains certainly won't stop).

All trains on this route do stop at Llandudno Junction - which is 1 mile east of Conwy.

At that time of year it is vanishingly unlikely you will be the only person either to alight or board.
Trains enter the station at walking speed, expecting to stop.
Also the station is only two cars long, up to 5 car trains stop there, so check you are in the correct portion of the train.
To be honest consider using Llandudno Junction station, 15 minutes or so walk or a very short bus ride across the bridge. Everything, even the express services to London stops there.

That could buy you more time, especially as you can't walk off the ship- cruise or ferry. You have to take a shuttle bus from ship to passenger terminal/railway station.

I take it that you are aware of the combined ferry and train ticket known as SailRail?
https://www.irishferries.com/uk-en/Travel-to-Britain-Specials/sail-rail/
https://tfw.wales/ways-to-travel/rail/ticket-types/sailrail

I've just been looking at the "printed" timetable. The fact of Conwy being a request stop is out of date.
All trains scheduled to stop there now have it as a compulsory stop.
For general information in printed timetables request stops in Wales are indicated by an 'x' next to the station name.
In England and Scotland the x is between the hours and the minutes in the timing column.