They are adding DUB AMS to penetrate more of the Aer Lingus network. Simples. AMS may be a bit different to BRU for them as Aer Lingus has strong feed to KLM from that route, where as BRU wouldn't have nearly as much, but it has other stuff like government ministers etc...
Aer Lingus will do a good job holding its own and with growing transatlantic feed the AMS route will survive..