Port Police can be a bit anal, it's private grounds still. If you had trade plates, that would help, but a bit dodgy unless you have the bloke who owns them to come with you and drive the car so it's legit. But that's down to no tax or MOT like mine was.

I'd just hire a flatbed and ferry to and from Ireland, that way you use diesel only, no wear and tear on the car and reduce the chances of breaking down (Ford or not ).