I'm not an expert, but as far as I know, there are two ways an open-jaw flight like DUB-XXX-MAN can be priced:

If the airline has filed two return fares DUB-XXX-DUB and MAN-XXX-MAN which allow combination, then the total price can be 1/2 of DUB-XXX-DUB + 1/2 of MAN-XXX-MAN (roughly). So if MAN-XXX-MAN is much more expensive, DUB-XXX-MAN will also be much more expensive.
Alternatively, this can be priced as two oneways, DUB-XXX and XXX-MAN. Oneway flights are typically more expensive than return flights, so then it would also be logical that the price for the open jaw flight would be much higher.