I think March 1991 was when BA pulled out of the Republic of Ireland, their eventual return was via a BA franchise, CityFlyer Express who flew ATRs and later RJ100s from their base at Gatwick on behalf of BA. This route then passed to BA itself when they bought out CityFlyer but they couldn't make it pay with BA mainline (again) and it was dropped at the start of 2009. One of the problems of BA LGW moving to only B737-436s was it's too much aircraft for some routes.
BA CitiFlyer also operated a short lived LCY-DUB run from 2008-2009. This was axed due to CityJet's holding their own on the route with CFE having poor loads.
BA do still code share on Aer Lingus services into LHR and LGW though.
BMI reduction on the LHR route was because Lufthansa reduced the BMI fleet by 5 aircraft.
They dropped AMS and BRU at the same time, it's not just a lack of capacity, it's a shift in business model.