If you are connecting from BOS-DUB-EDI, it would be strongly advisable to book Aer Lingus as they will transfer bags at DUB. FR and EI use different terminals at DUB so if you were transferring yourself there, it would be rather tiresome. EI will also honour your transatlantic baggage allowance, whereas FR would require you to pay baggage fees.

If you book EI on the 21st and misconnect, I don't quite know how they'd handle it on separate tickets. You would be at risk of being boned. So do book the night before and stay at the airport. I normally recommend the Hilton although it's not on the airport campus; the Radisson and Clarion are the only hotels genuinely at the airport.

FR is very reliable these days and generally very punctual. They don't cancel flights at random anymore because they'd have to pay EU compensation of €250 per pax or more. However, any failure to meet any of their arbitrary conditions results in swingeing surcharges (think Spirit). This includes strictly one item of hand luggage (and they really do mean one; there is absolutely zero acceptance of a personal item, whether it is a handbag, camera, shopping, or otherwise), which must fit the sizer (which is marginally smaller than many other airlines) and weigh under 10kg (not as carefully enforced). Failing to check in online also carries a penalty charge, as does failing to have your boarding pass stamped by their passport checker if it's a non-European passport. If you're late to the gate you're liable to be offloaded and be charged a walk-up fare for the next flight.