Indeed the Irish market was extremely seasonal until about the 90s when DUB started to develop as a financial and tech hub. Ireland was broke until then obviously which limited outbound demand to emigrants and non revenue passengers. EI had 3 747s until circa 1994, they were often rammed in the summer and an additional widebody would be leased in from Caledonian or World, whereas one of the 747s would then be leased out for winter months, Air Jamaica and LAN Chile certainly had them at different points.
When the Irish economy was particularly bad, a 747 would go on longer term lease elsewhere, one was at BA for a few years in the 70s. The EI executive who came up with the leasing concept (Tony Ryan) went on to found his own leasing business, GPA (now GECAS) and became a billionaire out of it. He also founded a certain rival airline that still carries his name.