Can you really see Aer Lingus ( Ryanair ) at LHR for long sitting on the slots that are worth a small fortune? This policy is incoherent but they don't care as it's no secret what FR really want to do to EI.
You stop at SNN on way from the US and get bags cleared plus do immigration and land 2 hrs later at LHR as a domestic passenger with no delays or queues and likewise going back.