I'd have thought the 707s would have had some planned, regular work as well as sports charters and back-up duties. As said upthread, Mediterranean holiday stuff could well have provided the bulk of their work. I don't know what sort of market there was in Ireland for longer-range holidays abroad in the 70s and 80s but the 707 should have been able to manage the distance.
Two things have historically struck me about Aer Lingus aircraft. First, whenever I saw one, it was always spotlessly clean on the outside ( engine exhaust dirt notwithstanding). Second, the aircraft were seldom idle. If they weren't earning money for Aer Lingus directly, they were working for somebody else and earning brass that way.