To a certain extent, browning leaves on bearded iris during the summer months is just a fact of life. The older leaves will always turn brown and dry up regardless of what type of care they receive. You can pull or trim off these browning leaves as they develop to keep the plants looking tidier.
If the browning problem is severe, you may have a fungal leafspot problem on your irises. Do they first develop yellow or brown splotches before turning completely brown? If so, spraying a fungicide may help prevent the excessive browning.