Is this because Irish provinces, being IRFU owned, are able to manage players better? I'm assuming that if central command were to stipulate that, say, Sexton were only to play 15 games a season (roughly equivalent to what the WRU requires Cardiff to do with centrally-contracted Warburton) then Leinster would make it so, a luxury that the RFU and, to a lesser extent, the WRU, would be unable to do lest it bring them into direct conflict with privately owned clubs.
It's no coincidence that seven out of Wales' ten key injuries were all players who toured NZ in the summer. England have lost Sinckler and Daly from the same crop, you guys SOB and Payne. Given the fact that Ireland and Wales contributed roughly the same number of players and test starters is this coincidence or a difference in how players' seasons are structured to give them more time for recovery?