I have a feeling I know the answer, but I'm hoping otherwise...

I booked a round-trip flight over the New Year (MSP-ORD-DUB) on Aer Lingus back in July. Due to a recent change in plans, I need to return to BOS rather than MSP. Expecting to deal with customs the typical way in ORD, I decided to buy a one-way ORD-BOS on UA thinking I'd throw away my last ORD-MSP segment (also UA) on the return.

Except it sounds as though there's preclearance in DUB for most flights to the US, meaning even if I decide to hop on ORD-BOS, my bag will still get sent to MSP. What time is the cutoff for the DUB preclearance? How much will this cost me if I call the airline? Suggestions appreciated.