on a wednesday there is a spare A332/3. The aircraft normally operated DUB -MCO 3 days a week and the 2nd daily flight the other 3 days a week leaving wednesday as a free day, normally this allows for maintenance to b carried out or flights upgraded because as u say, FAO is sometimes booked at capacity. Up until the EI/United JV it used to be operated by the A330 as well as AGP and sometimes NCE.
The long haul fleet are currently operating as follows........................
1xJFK 104/5 Daily (the aircraft that comes in as EI104 normally rotates and does the AGP flight and then the 108/9 to JFK)
1xJFK/AGP 584/5 and 108/9 Daily
1xBOS EI139/138 Daily
1xORD EI124/5 Daily
1xSNN (operates m-w-f-s to JFK as EI110/111 and -T-T-S- to BOS as EI134/5)
1x -T-T-S (normally an A332 to MCO then on M---F-S to BOS as EI138/9)
1x IAD-MAD (UA Joint Venture - A332 -now operated by EI-DUO, was EI-DAA up until the 12th July but aircraft switch was made in BOS)