I'm sure the operators at any major European airport would kill for it, but there's a large amount of red tape and political complications. U.S. Pre clearance is technically U.S. soil, same as an embassy really. It's all sorted at government level. I have no idea why SNN and DUB are the only airports in Europe with it, but it's a dream come true for those 2 airports and the airlines/biz jets using it. Pretty handy from a pax standpoint too.