The reason why the Irish celebrate St Patricks Day more so than the English celebrate St George's Day is (in my opinion) partly cultural and partly corporate.
When you have a corporation like Guinness behind something, it's going to be big. And culturally - it's been going on for so long now, that it's become culturally ingrained...like the Mardi Gras, Glastonbury or the Notting Hill Carnival - if it didn't happen one year, it'd be noticed.
I'm not Irish, but I know exactly when St Patrick's Day is, because it's such an obvious day each year - our own saint's day isn't so obvious, because we've not got a big corporation ready to relieve us of our money in order to celebrate it.