I never said anything about ORK-LHR being the second busiest route! Unfortunate wording by another poster makes it look that way, when it was in fact sad by him!
The 5th daily LHR-ORK has been on/off in the past few years as has the A321 on the route. As I recall it was operated by a DUB based A321 for a few years, with the other 4 flights by ORK based A320s. There was an A321 based in Cork - around the time SNN-LHR was suspended until recently, although that ended when half the A321 fleet was retired this winter. I think since the return of SNN-LHR the fifth ORK-LHR has been used to add a fourth daily into SNN for the summer since the return of that service.