There was schedule work for one, maybe two in high summer, 707s after the 747s came, but there was a lot of spare capacity. Aer LIngus ran a surprising number of 707 charters originating from the UK to the USA during the 1968-78 charter boom, before ABC flights consolidated things, there must have been some licence condition that permitted them to share this traffic.
The 747 always seemed far too large for Aer Lingus at the time, certainly for 8 months of the year, and it's surprising they never downsized from them.
The One-Elevens were bought for, and for quite some years monopolised, not the trunk route from Dublin to London but the low frequency continental services, many of which stopped in Manchester to pick up additional business. Some of these only ran a couple of times a week, while in their early years the Heathrow route continued with high-frequency Viscounts. They only bought two 737s in the late 1960s to begin with, and so the Viscounts continued to supplement them for some time until more trickled in, often just one at a time. In fact throughout their time in the fleet the One-Elevens were unusual in Heathrow.
There was one exception, from the start the early evening Dublin-Liverpool flight generally seemed to be a One-Eleven, after a day flying to Europe and back. As a teenager it came overhead our house in The Wirral, making that distinctive One-Eleven Descending aerodynamic noise, and I would look up from my school homework to see it pass. Fast forward through decades of life, and at the start of the 1990s when making business trips to Dublin, with colleagues who were probably not even born when I was initially watching, there were the same One-Elevens, with the same operator, setting off from the same base. Only the paint colours had been switched round a bit.
Aer Lingus also got epic service out of their Fokker F-27 Friendships.
I beg to differ. Although they bought 7 of the very earliest Fokker F.27s in 1958-9, they sold them all off just seven years later, a number to New Zealand, and replaced them with secondhand, and in fact older, Vickers Viscounts from KLM, to give a standardised short haul Viscount propeller fleet. In fact, they were some of the earliest secondhand F.27s on the market.