Need advice about picking a car up in Ireland, dropping it off in London or Glasgow

Use two cars.

The ferry companies don't give a rat's rear bumper what you do. The rental agencies care big time - - taking a car across the Irish Sea in either direction voids the insurance. Some of the majors (maybe just Hertz ?) make an exception, but stiff you about three hundred bucks a day for the privilege - - normally you'd expect to pay twenty bucks a day for a small car in either nation (plus the extra for the Irish insurance on that end).

Any British departure point would work, depending on exactly where you're going. You could throw Manchester into the mix as well.

Ed has it right - two cars.

The fine print on our rental agreement last year (Ireland) clearly stated that we were prohibited from taking our car across by ferry.

I've always done two rental cars when I've traveled over there, but I often wondered if there is a better deal if you rented the Irish car in Belfast and then brought it over the Irish Sea and dropped it off over in London. Has anyone ever looked into that?

You would pay a hefty fee for this for insurance and the cost of bringing the car back to Northern Ireland. Car registration, safety tests etc are completely separate in Northern Ireland from the rest of the UK so the car has to be brought back.

We agree with the advice to rent two separate cars. (I took my car on the ferry when it was leased for several months, but the rules for regular rentals are different.) You can't ignore the fine print on the rental agreement. Luckily, Auto Europe has made their fine print easer to read in the past couple of years.