If EI decides it wants to charge you and your bags were interlined, you will be paged at the gate at LHR. I've seen/heard it done before.
However, if it's a return journey, you will not get away with 20kg excess in DUB. EI has allowed me very considerable latitude in the past (16kg excess across two passengers is the best) but having double the allowance really won't wash. (Try booking the return journey direct on EI perhaps?)