Will breastfeeding diminish my iron supply?

Yes. Yes, breastfeeding provides nutrition to the infant, including iron. If the mother isn't replacing the iron the infant is taking she will be at risk of becoming iron deficient.
No. Breastfeeding does not decrease the amount of iron supplied to the breastmilk by the mom's body. This means the mom's body continues to put iron into the breastmilk to benefit the baby, even if the mom herself becomes iron-deficient. So, the mom should eat enough iron and/or take some iron supplements, so she maintains adequate iron for herself..
No. Not dramatically. There is some iron in the breast milk, but it's not a large amount. It's increased bioavailability puts it on par with formula, but the actual molecular amount is less. You should still take a multivitamin and eat well to replenish what the breast milk uses, but the amount of iron lost is not dramatic. If your iron is near concerning levels, make sure to consult your doctor.