as andy mentioned, kirsch is a cherry-flavored liqueur, so it flavors and moistens the cake at the same time and, being alcoholic, gives it a slight "adult" taste.
first off, don't worry if you don't have it. cooks always work with what they have. sometimes that means that a dish may come up lacking, but sometimes it leads to inspiration.
o.k. so this cake is all about chocolate and cherries together. if the cake is chocolatey and moist, and the cherries plenty and flavorful, the lack of kirsch won't be missed. if the cake is delicious by itself, i wouldn't even recommend using a sugar syrup to moisten it. that's more of a way to save an overbaked cake or otherwise dry cake.
if you have canned cherries on hand, then your problem is solved. just use the liquid from the can, maybe mixed with a little brandy or something.
if you have some dried cherries, some vodka, brandy or other not-too-strong-flavored liquor, and over a month, you can make a kirsch substitute. pack some cherries into a small jar, add a handful of sugar, top it off with the liquor, and let it sit for a month (or more; usually a lot more). if this sounds rather imprecise, i'm sure you can google making home-made liqueurs.
or how about this? if you have some cherry jam, thin it with some brandy and then brush the cakes with it. similar to a sachertorte.
it sounds like you've got plenty of time, so you'll likely be able to come up with other ideas.
best of luck, and let us know how it comes out