I read an interview with Norwegian CEO in this months Airliner World magazine where he states (words along the lines of) that they were in serious talks with MAN about a long haul base but have withdrawn from these talks as there's too much competition at MAN and that they're now looking to set up a base elsewhere in UK instead (in addition to EDI).
I've given up trying to understand what Norwegian are planning as it seems to change on a daily basis! Interesting that the article suggests setting up in elsewhere in the UK which would rule out DUB? Again, what Norwegian say and do aren't necessarily the same thing.
I'm not surprised though by the statement that they walked away from MAN due to too much competition - they were all set to set up shop at BHX until Jet2 came along and then walked away from the negotiating table. Perhaps they do understand that there are limits on what routes can be viably served from the UK.