FR 3977 then FR 7808 at Barcelona
Dublin → Barcelona → London Luton
Timetable, Barcelona time
21:10FR 3977 from Dublin at the gate
12:45FR 7808 to London Luton leaves the gate
15 h 35 apart on the timetable, overnight
FR 7808 leaves 15 h 35 after FR 3977 arrives on the timetable: more than half a day, so not a connection we count.