Whether they are bulbs or rhizomes plant them outside now.
If they are rhizomes follow instructions here http://mrbrownthumb.blogspot.co.uk/2009/10/how-to-plant-iris-rhizomes-in-your.html
it's a bit late but they are living things and will not survive if you do not plant them now.
If they are iris bulbs/corms (like little daffodil bulbs) plant them now following the instructions here http://www.crocus.co.uk/plants/_/bulbs/other-bulbs/iris-harmony-reticulata/itemno.BU30000721/. Again, if you keep them they will deteriorate over the winter and die.
It's only a few days past September, the plants can't read the calendar - better late than never in this case - but definitely plant them in the autumn, not the spring. They need to start growing now and experience the cold winter in order to make them flower in the spring.