I had also heard the myth about Croke Park, so I took a look, and there are structures that look somewhat like platforms, but I think they are just concrete supports to hold the stadium up. The clearances are just too tight for passengers to safely disembark trains, and there's no way you could fit a station in there.
If Metro North does go ahead (and I believe it has to eventually), I think the ideal place for an interchange station with the Maynooth line would be further west of Drumcondra near Phibsborough. It would seam feasible to put a station near the tennis courts on Whitworth road, which would allow both train lines to connect with the Metro platform. This would be a minor alteration to the current route, and would allow the station to be built using cheaper cut and cover methods, rather than more expensive mining methods that would be required in Drumcondra.
I sometimes wonder if we are putting the horse before the cart expecting existing high density population centres before we consider a rail connection. Inevitably the centres we are trying to connect are too spread out to make the best use of the rail connection that eventually gets built. If we built the rail line to Navan, and zoned a number of areas along the route for medium to high rise housing, the population centres would follow.
I suspect nothing is really change until the cosy political cartel of FF/FG gets broken.