For how many years did Aer Lingus manage a Split Run (by which I presume you mean flights originating in DUB and BFS meeting in SNN to swop onward pax to JFK and BOS)?
I only recall using it once.
For me, transatlantic from DUB is a no-brainer, although I did occasionally depart DUB and return to BFS (Continental/UA) to have local immigration/customs and short journey home on return
IMHO, there simply is no sustainable BFS transatlantic market other than a few summer holiday jaunts.
I think the carriers have established that as a fact