[strike]A steel frame usually uses a 29.8mm collar but there are odd ball frames.[/strike] I have a good feeling yours uses the standard. Only one way to be sure through, measure.

EDIT: Just remembered IRO did make aluminum frames, I was on the assumption they only made steel frames with their end of life stock... a good ballpark guess would be it uses a 27.2 seatpost if I remember correctly which should mean a 31.8 collar; what live wire said.