I've not been able to a find clear answer on this, so any help appreciated.

I'm flying an exEU return SEA, JFK, LCY (baby bus), DUB.

I'm aware the short checking policies are unlikely to allow me to check a bag from SEA to LCY only, however does the transit process at LCY negate that? Would I need to collect and recheck at LCY for the DUB flight anyway?