Tom,
You wrote that "quite a few courses in Ireland have recently made their courses longer and more difficult, to try to attract Americans to pay higher green fees to subsidize the locals".
While I don't disagree that courses have been lengthened, I'm of the opinion that magazine rankings have a bigger part to play when it comes to clubs outside the south-west. The more isolated clubs very much depend on Irish visitors, and maintining a high position in the rankings is one way to promote their course. I've seen clubs in the GD Ireland Top 100 list displaying their ranking very prominently in their clubhouses. The Kerry and Clare courses are very dependent on American visitors, but I don't think American visitors are a big factor as you head north of Galway, especially not in the north-west.