Kirsch, according to Cook’s Thesaurus, consists of a cherry-flavored brandy (see reference 1). It appears in some recipes, but some do not want to use alcohol in their cooking or cannot locate this drink. Substitutes exist for Kirsch liquor, but as with any replacement in a recipe, the results might taste slightly different. According to Gourmet Sleuth, the official alcohol used in fondue recipes is kirsch because the alcohol aids in smoothing the melting of the cheese (see reference 3).
Syrup or Juice
Replace kirsch with an equal amount of black cherry, raspberry or boysenberry juice or syrup (see reference 2). Either drain the liquid from cherries canned in syrup to get cherry syrup or look for Italian soda syrup in a cherry flavor (see reference 3). Only use syrup or juice in recipes where the alcohol does not play a chemical role. For instance, avoid using non-alcoholic substitutes in fondue (see reference 3).
Framboise or Cherry Liqueur
Substitute Framboise, a French raspberry brandy, or cherry liqueur in the same amount (see reference 1). Use Framboise for if your recipe depends on the brandy flavor, but opt for a cherry liqueur for more cherry flavor in your dish.
As with using juice or syrup, you cannot substitute cherry preserves for kirsch in fondue recipes, but they will replace the cherry flavor in other dishes such as baked goods (see reference 3). If desired, thin the cherry preserves to a watery consistency with boiling water before adding it to your recipe to more closely replicate the texture of the kirsch liquor.
References and ResourcesCook's Thesaurus: Brandy
What's Cooking America: Alcohol Substitutes
Gourmet Sleuth: Kirschwaser