Direct access to controller
The docs explain how to generate links to controller actions. Assuming your action trigger is "actions" (as it is by default) then your URL will look as follows:

http://www.domain.com/actions/pluginName/controllerName/actionName


Following the first example in the docs, the URL will look as follows:

http://www.domain.com/actions/cocktailRecipes/ingredients/deleteIngredient


Be sure to allow anonymous access to your controller action so it can be reached by the cron job.