Like an earlier poster I remember seeing them as far back as the green stripy EI livery as one of the Sunday 'oddities' that LHR seemed to have back then.
I too thought they went to Cork or Shannon.
I remember turning up at Dublin to find a BA 1-11 500 once which was bit of a surprise but much later on found myself on EI 767 so it was route which had bit of variety from time to time .