Welcome to FT srmrkng!

Firstly, unlike the US you would not have to collect your luggage at LHR and go through customs/security with it for a connecting flight. It would be checked through to DUB, unless you have told them otherwise at IAD.

Secondly, if this is the outbound leg of a return trip bought on the same ticket, then when you skipped the LHR-DUB segment all subsequent segments would be cancelled and you would get a nasty shock when you turned up for your return flight in DUB. However if this is the return leg or a one-way ticket and the LHR-DUB flight is the final segment, no harm in ditching it (making sure your luggage has not been checked all the way through to DUB of course!).