80K for England and France every 2nd year - fair enough - as long as the team is being competitive. I've been at games in the bad old days and you could swing a cat on the North Terrace.
80K for Scotland and Wales ? Remains to be seen. You're dependent on the fair weather fans and the event junkies. The Heineken Cup can be a better day out so if you're choosing to allocate funds against a day trip to Dublin to see Ireland V Italy or travelling to see Munster playing in Castres or Biarritz of Toulouse, Dublin ain't the draw it was.
IRFU would not fill an 80K stadium 3 times a year every year .. IMHO.
Back on topic - I listened to the podcast of the original discussion and I can't get my head around the relative heights. Croke Park, from my vantage point of the DART line, seems huge and I cannot imagine something needing to be higher than that, for a smaller capacity Equally I have been in Cardiff and the Millennium Stadium seems, from ground level, to touch the clouds.
As I have said before, the legal profession will have filled their boots befiore the planning is granted for Lansdowne Road.