it would be better to hire a 7.5 ton, rather than a luton 3.5 ton, and to do it in all in one trip. anyone with a car licence older than about 5 years (im not sure when the regs changed) should be able to hire up to 7.5t (class C1 on the license)

you would be better to hire a 7.5 ton from cambridge, drive that in tandem with james car, both driving to wicklow. then on the way back park james car at dublin airport, and both come back in the van. on return to cambridge both you and he get a cab or a lift to stanstead.