I've used the left luggage service at FRA and it is very professional. Better than any hotel that I've seen. They even xray all luggage prior to accepting it for storage. I would have no qualms about leaving it their for any duration, it is safer than with the airlines! Dont remember the cost exactly, it may vary with size and is about 5 euro/day (so your week storage will cost about USD 50 or quite a bit less than Aer Lingus would charge for excess baggage).