As for Portadown being the location, I ask why not. Is there a suitable central location that was available, affordable and overlooked?
Garvaghey, Owenbeg, Cloghan (or Inniskeen which is really a training facility??), Convoy, Ballykinler, Lissan, are not exactly central or at their county grounds. The only exception I can think of in Ulster is Cavan who are developing at Breffni, but then they have a lot of land there available to them. Antrim don't even have a home ground at present and there's certainly no room beside Casement for a development like that.
Our other neighbours in Louth have an impressive set up at Darver. May be half way between Dundalk and Ardee, but not handy for some pats of the county, nor near the central location of the hub of Drogheda.