The first time I played Waterville in '96 it was raining the whole round and I'm told it had rained a lot there in the preceding couple of months. The course played like a bog and I hated it. I returned in 2005 and it was a windy but dry day, and all the courses in SW Ireland I played were pretty firm and I was able to enjoy and appreciate Waterville that time. I don't recall Waterville lacking in any firmness, though it certainly did not play as firm as say Lahinch or Ballybunion.
Given the land features around the course I suspect that it is not true links ground so it is not built on sand, or is at best a sand mixture or sand cap. I think the Ring of Kerry is supposed to be the rainest place in Ireland, which is a country that does not lack for rain. Given Waterville's position at the western edge, it is probably one of the wettest places in the Ring of Kerry, which would make anything other than true linksland with a deep sand base difficult to keep fast and firm.