I retired from painting after 30 years. At times we needed to offer a surface additional protection from moisture and/or dirt. We used an acrylic (water-based) varnish available at most paint stores. It comes in a variety of sheens. The best to use to reduce glare is a FLAT sheen. This stuff isn't cheap but hey....what is?
The surface to be covered must be thoroughly dry. I assume you used acrylic paints on the mural. Let it dry at least a few days. Test a small area first to make sure the
acrylic varnish doesn't have a negative effect on the mural.
Apply the varnish with a brush and a small (4"-6") roller.
Do not overwork any area as it could "wake-up" the paint of the mural and make a mess of it.
Sometimes an additional coat is required to give complete coverage.
Clean up tools and self with soap and water.
Good luck!