Who are you trying to book with? Irish ferries have a 'slot' for vans minibus etc over 8m, I've also booked with Stenna on-line (our bus is 7m), more crucial is the height you usually get to go on with the lorries etc which is great as you don't have the ramps.
You don't need a passport, but if you have one it's a good idea to take it with you.
All the Irish sea crossings are very expensive,5 of us 2 adults +3 kids usually cost around £350-400.
I tried a few discount booking places, both stenna and Irish ferries are coming back at about £140 one way, but they are only going up to 2.5M in height, so no good for me either.
My passport expired a few years back and I didn't bother renewing it as we don't go abroad any more, or should say didn't go abroad. That may soon change if I get the van
As tinker says, you don't need a passport (assuming you're British or Irish) but the ferry company may require identification. As you will no doubt be taking your Driving Licence anyway, this should be adequate.
If you book on the ferry operators websites I think you will find that they have a slot for over 2.5m, if not try the van category. Basically it just means that you go in with the trucks & headroom is OK. Once I found that Freight rate was actually cheaper than Motorhome.
Last time we went to Eire, I booked for the motorhome which was 6.1 metres. On St Patrick's Day, Irish Ferries did a half price thingy so it was £170 return, Holyhead to Dublin.
Then before we went, I sold the motorhome and bought a folding camper, which I towed with a Shogun - total length over 7.5 metres. They didn't charge me any extra for changing it.
------------- Caz
If you can't see the light at the end of the tunnel, just keep going till you go round the bend.
Think you will need photo ID, not just the paper bit, or an old green driving licence, would hate you to pay all that money and then be refused travel!!