In addition to treating hardness, water softeners are frequently use to reduce small amounts of iron. This isn't a bad plan, but sometimes it doesn't work well. Here's why.
A water softener is an ion exchanger, not a filter. Soluble iron (ferrous iron) is readily taken up by the softener resin, but "precipitated" iron (ferric iron) often slips right through the resin. Another way to look at it is, if the water is clear, the iron will likely be removed by a softener, but if it has a rusty color most of the iron will pass through the resin.
The most common determinant is pH. If the pH of your water is below 7, a softener will likely remove iron well. If it's over 7, results may not be good.
If the pH is above 7, you need an iron filter, not a softener, to take care of the iron. Typical iron filters use such media a Birm and Filox. They do not treat hardness (excess calcium and magnesium), so you may need both a softener and an iron filter if you have hard water with some iron in it.
If you use both an iron filter and a softener, the iron filter goes first.
(whole house & well units)