ROSCon and IROS are septate events, and to my knowledge there are no registration dealings between them. However, you can register for just the tutorials and workshops for IROS, if you do not plan on attending the main confrince, and is significantly cheaper than registering for the whole event. As a reminder, IROS does have discounts for IEEE members, and the at-the-door registration will cost more than if you do it online before hand.