Open tickets on Ferries?

My wife and I are coming over to Ireland next month via ferry. We are undecided at the moment how long we will be staying and I would like to know if it is possible to book open tickets on the ferries to/from Holyhead.

I emailed both Irish Ferries and Stena but so far have not received any response from either.

I would appreciate any help with this query.

Thanks

BillK

I remember turning up at Holyhead a couple of times a day earlier than anticipated and just handed in my ticket with the next days sailing date on it and I didn't have any problem. I was just waived on board.