The gradient is hardly that scary compared to what some light rail lines do in Lisbon, Zurich or Oslo and besides, it wouldn't have to head straight up the hill, going up through Grange would significantly reduce it.
As I said in my original post, the economics aren't there at the moment, but whatever about the cost, the important thing is to identify an alignment that wouldn't be built on so that if the economics were there in future, it wouldn't mean extra spending CPOing buildings that should never have been build. We're not very good at planning for the future in Ireland, but that's no reason that we shouldn't start.