I think people take this supporting England thing too seriously. Why on earth should we support England, or any other country for that matter? England sure as hell don't support France, even though they are geographically far nearer than we are to England. France don't support Belgium. Last time I looked, Australia don't exactly cheer for New Zealand in any sport. It's healthy rivalry and schadenfraude (sp?), nothing more - or following Purple's argument should Arsenal fans cheer Chelsea if they make it to the Champions League final against say Real Madrid just because they're from the same city? Wallowing in another team's sporting misfortune is part of enjoying sport, as much as your own team winning - sport is after all primarily about rivalry.
There seems to have developed a certain orthodoxy in Ireland in recent years that if you don't support every England team as soon as Ireland is not a factor in any tournament / sporting event, that you're some kind of unreconstructed bigot. I reject that utterly - even if you don't agree with what I say above, many Irish people don't support English teams/athletes because of the nauseating jingoism of their sporting media.
I don't automatically support England but in the case of the RWC I admired how they exceeded expectations. I also admired the fact that they were oldest team in the tournament, with the oldest player ever to play in a final. Therefore I supported their team when they played against South Africa. I would have cheered for Argentina if they had made it to the final against England as I also admire their team.
As Seagull pointed out, there is a bid difference between supporting English teams as a matter of course and childishly cheering for anyone they play against.
As for the "nauseating jingoism of their sporting media", no one is forcing you to watch British TV or read British newspapers. Try the Irish media for a change.