The 19-year-old striker has made the first-team breakthrough at Bramall Lane this season, making 11 substitute appearances for Danny Wilson's side.
"Joe has done very well this season and taken advantage of us having a limited number of striking options," Wilson told the club's official website.
"He has filled a void very well and that enabled us to let Chris (Porter) move to Shrewsbury Town on loan.
"That is a credit to Joe that he has proved he could step up to the challenge if required.
"Let's hope he can develop into the very good player we expect him to."
Ironside, a member of the Blades youth team which reached the FA Youth Cup final in 2011, made his first appearance in the Johnstone's Paint Trophy win at Notts County in October and his league debut at Carlisle in December.