The Leinster back's call-up comes after Reddan picked up a calf problem in training on Tuesday.
"We've called Isaac in for the session this morning because Eoin has a slight problem with his calf," assistant coach Les Kiss is quoted as saying by Sky Sports News.
"We'll give him until the end of the week and see what happens. It's very unlikely that he's going to be there."
Ireland welcome Scotland to the Aviva Stadium in Dublin on Sunday.