Grit and dirt in well water is pretty normal. It is reduced with a filter. In this case you need a changeable/cleanable filter on the main water line between the well and the irritation system. That is not necessarily the landscaper's job. It could be the irritation system installers job. This is not an unknown issue for irritation systems.

The contractor you need is a plumber if the people who installed the irritation system don't want to do it. How the infrequently used well is set up will determine where the filter goes.

Agree. This is not a landscaping problem. Get the well fixed.

I live in a community in Texas with its own water system. They had a recent change in management, and they are discovering lots of maintenance issues overlooked by the previous managers. They sent divers into the water tanks to check for sediment and found 4 inches of rocks and sand. The next month they sent the divers back to check for new sediment and found plenty. They are now on a monthly sweep schedule to clean the tanks.