Each team is entitled to having one IR Spot. In keeping an IR player from your current roster, you must make a post to the league that you are putting a certain player on IR, then drop them. Make sure they're actually on IR first, or you lose all rights to them.
On IR meaning that Yahoo identifies that player with an IR tag, if not, it will not apply.
Once you have posted your intended IR player to the league's message board, you own the right to that player until the beginning of next year's draft. Although the player is no longer on your final roster at the end of this season, as long as you post to the league, that IR player is entitled to be one of your KEEPERS for next year's roster. All draft position rules still apply to the IR player.