If you have a checked bag to collect you can't really optimise this. The connections process at Dublin is to go from the bag collection area directly upstairs to security with. That only skips the customs door and the first half of the security queue. It doesn't save very much and it doesn't give you any way to check a bag.

I don't think BA will check your bag through back to London, I suppose there's no harm in asking though. In theory you could show up and check in for your longhaul and check your bag directly onto that but I don't think they'll accept bags more than something like three hours before your flight.