Firstly, many cultures prefer to give children names of ill omen, on the assumption that any jealous god passing by will ignore them; if they had names with beautiful, desirable meanings, the jg might see them as competition! So, given those presuppositions and the possibility of glossing Brendan as either Stinking Hair or Prince, the Stinking Hair meaning becomes pretty possible. Point is, what the ancient Irish assumed their god(s) would be likely to do, and that I don't know.
As for Katharine, lots of words look like other words that they've got nothing at all to do with. And if a name like Katharine existed and so did the word katharos, it would be easy to assume that they were one and the same. It wouldn't be a matter of inventing a name to fit the word. (For instance, where I live it is perfectly respectable for the speakers of some local languages to name their daughters Nakedi. This wouldn't happen in other parts of the world, and if it did, people would easily draw the wrong conclusion.)
Plus, the ancient Romans were total klutzes at etymology, and capable of just about anything!