Can you freeze mirepoix?
Yes, you can freeze mirepoix, but it'll be pretty soft once defrosted, as Peter V noted. Onions and celery do not freeze well, though carrots do okay.

All three vegetables, when whole, should be blanched for a few minutes before freezing. For vegetable-specific freezing instructions see the National Center for Home Food Preservation's Freezing Guide. 

It's best to use fresh, but blanched for 1-2 minutes then frozen works okay. I do not blanch diced vegetables for more than 2 minutes.