We are going to Ireland later this year (Holyhead to Dublin) and looked at prices and realised I could use my Tesco vouchers. Not amused to find that if I use Tesco vouchers the price goes up by £20 each way ! Hardly seems fair/legal/moral somehow.
I've just rung Tesco and they say that some companies do add on a premium if using clubcard vouchers and they cannot stop them doing so. It is mainly hotels, ferries etc which do it. I suppose it isn't quite so bad if you are using lots of vouchers but if only a few would hardly be worth it. I still think it is rather devious but hey ho that's life and not worth getting my knickers in a twist over. I did try ringing Irish Ferries but it is a premium rate number and didn't think it was worth the bother!