[Formendacil]

Perhaps Ivrin was simply a very "clean" place, very pure, unstained, and unblemished, but nothing more than that.

The Silmarils, the barrow-swords, the elven cloaks, lembas, etc., these all had some sort of power imbued in them through their makers, which made them anathema to evil. Need Ivrin have been the same?

After all, Ivrin is never actually said to "repel" evil in the way that any of the above do.

So here's the basic thought I've got going: Ivrin was a place that more or less "untouched" by evil, and was thus very pure and clean, and therefore very soothing to those coming in from a much more stained world. However, Ivrin had no inherent "resistance" to evil of its own, and thus it did not take much for Glaurung to defile its waters.

Just my two cents worth....