Well, yeah it is fair. It's part of the challenge, and if you take that handicap away, it's going to make it less challenging, less fun.

The boots wouldn't have worked either way, because the arena effect was to be made Sap. I don't know if Cuchulainn actually gives you sap himself..

It fits in with Cuchulainns history really. He used to be a really handsome person, apparently, absorbing all the bad things from people, but there were so many impurities in the world, that it turned him into..oogie boogie.

Kinda makes sense for him/arena effect to be zapping at your health, and to not be stopped by boots - one reason being, instead of being good and absorbing impurities, he's gone "bad" and is absorbing precious life/health. He is supposed to be like a powerful being, so some boots aren't going to stop something he's causing. Quasimodo boots would only block weaker creatures from attempting to use sap.

My contention is if they want a boss that has effects you can't block, make the party fight them before the means to block them is available. Of course that would be hard to do with an optional boss, unless it dropped said equipment.
But what if more then 1 optional/storyline based caused sap? Say, the first boss, then a last boss?