As savoryk says I leave out the kirsch and use white wine instead. It's just a personal preference. I always have a white and a red cooking wine open next to the cook top.

I have to confess, I have always left the kirsch out of my fondues. I know it's not traditional, but I like the results. Also, I tend to use dry vermouth instead of white wine. I seldom have open bottles of white wine around to cook with, but I always have dry vermouth around. I find any herbal notes in the vermouth work well with cheeses.

Tawny port? A different kind of fruit note but goes well with cheese flavors.

There's already wine in the fondue, so it's not a substitute for kirsh. TasteFood suggested Calvados or Pear William. Calvados is certainly not authentic, but it goes well with cheese. Pear William or another clear spirit could work. And I've known people to just leave it out.

Maybe amaretto?

I bet Vermouth would be nice.

A dry white wine will work ok. German whites seem to work well.