I'm not arguing with the benefits of of low floor for Luas, and I was surprised to see Manchester using high platform trams.
However, for a Metro system, even a tram-based one like proposed for Dublin, the disadvantages of low-floor would seem to outweigh the benefits. Being able to inter-operate with Luas seems a very minor benefit, and if the Green line south of Stephen's Green was ever upgraded to Metro, all the platforms would need to be extended anyway, so why not just rebuild them a bit higher.
As far as not being metro, I think the key things that make a system a metro are total segregation from other modes of transport, and high frequency service. The precise vehicle being used shouldn't be that big a concern, providing you can make them frequent enough to keep up with the volume of passengers.
I think upgrading the Green line would be a mistake, because you would destroy the segregation with road crossings, and inevitably result in a much slower and less frequent service.
I would rather see a Luas every 2 minutes than a DART every 10.