The difference is though due to the domestic setup; The Irish setup works as a club - > province - > International system, with none interfering with each others plans or setup. In England however, the self contained club league feeds directly to the national squad, but no concession is made for competitive club fixtures being played at the same time as international training or test matches.
So while the Irish provinces are happy for their players to join up with the International team, in England the clubs can find themselves losing key players for important matches because they're only bench-warming for England (or worse, simply travelling as "backup" players). After professionalism kicked in and relegation suddenly meant more then just playing against a different roster of teams instead potentially meaning the club went under (see:
Orrell Rugby), or the difference between a championship or mid-table obscurity, clubs and fans alike didn't like the distainful attitude displayed by union towards them. This began the club vs country row and also why several clubs began to rely on imports instead of producing English players. The Clubs didn't like the idea of having players who the union could effectively ban them from using more then 15 times a season.
The Union, being the self-serving wankers that they are, wanted central contracts on players who they don't pay the wages of while the clubs told them to f-off. Eventually the EPS agreement was made so the Union pay a lump sum to be distributed between the premier-rugby clubs regardless of how many players they provided. This had 2 major faults however;
1. If a club provided 6 England players in one matchday squad (they could even provide all 22), they would still only receive the same amount as a club who provide none.
2. The loopholes in the system mean that any player who is injured can be replaced at a moments notice with anyone.
What effectively happened is the Union now pays less to the clubs for the EPS then they would for the centrally contracted players, but they now have more control and selection then ever before. The clubs and the regular season-ticket purchacing fans are the ones who were screwed.
This is why we've seen over the past few years Northampton become the reject-All Blacks, Saracens become the reject Springboks, LI become the reject-Fijians etc.