I think the "Real Ale scene" in the north is confined to a few pubs in Belfast and the JDW in Derry, the Crown and the John Hewitt are both excellent, the other GBG pubs are a bit variable from my limited experience and there's not much else for ale. Ulster is a nice place to visit anyway, drink whatever there is and enjoy the people and the scenery. Never been south but I believe it's mainly Harp or Guinness with the odd beacon in Dublin.