En Route Low Altitude Military Training Routes (or IR routes) are used by Department of Defense military aircraft for low altitude navigation and tactical training. There are hundreds of these routes in the United States, and they are listed in the DoD's Digital Aeronautical Flight Information File (DAFIF).
The specific routes listed can depend on the region and the DoD training requirements at any given time, but examples of some of the routes include IR 101, IR 105, IR 124, etc. These routes are typically marked on aeronautical charts and pilots are trained to avoid them unless they are part of a planned military exercise.