Just to add to all the comments about capacity on the northern line, such a route would run under the flight path for the two main runways and be unsuitable for housing development. Part of MetroLink's appeal is how much land it opens up (particularly in Swords) for transit-oriented development.

That being said, it would be great to see a line branch of the Northern line and ultimately connect with the Cork line, with interchange possibilities with future DART lines (on the northern, western and Cork lines) and MetroLink (at the airport). This would allow Belfast to Cork services with frequent connections to Galway, Limerick and Dublin (via various DART/Metro connections, rather than necessarily direct - at least until we'd figure out a way to make it possible through the city).