Aer Lingus had a very large advertising campaign in London for the Gatwick base, there was the 999 nudes publicity stunt, TV adverts which ran for a month in March before stopping in April and starting again in May. During this they also had a print campaign with adverts in newspapers, billboards and on many bus stops around London and the surrounding areas. Aer Lingus currently sponsor the Capital Breakfast radio show which is listened to by around 50,000 people. The print adverts are still running at the moment, they have them in local papers such as The Metro, Evening Standard, London Lite and even more local papers such as the Crawley observer, Harrow Observer and so on.
I haven't heard of many TV adverts in Belfast but I have seen billboards for major sales, radio adverts and a print campaign. Aer Lingus sponsored the Belfast City marathon and also have a schools art competition every year which gets their name out there.
I do agree that more should be done though, the Aer Lingus brand is strong in Ireland and with Irish communities in other nations but with everyone else they are probably unheard of so it's important their brand is put out there. They've done a decent job with London but that's just the start really.