Peacock1, if you read the post from Ryan2000, he did say prior to 1986.
You state 1990, which was a whole new world in terms of price. In fact the date u mention is only 2 year before BA axed the route, the price war was well under way with FR offering unrestricted one way fares that Didnt require a Sat night stay. Prior to 1986 the fares were commonly IR£300, with day returns as high as suggested.
This is well documented in many publications when EI and BA had cosy cartel. Google Dublin london route and cartel and you should get some evidence.