The Republic of Ireland international has completed his move from Derby County, with a fee reported around the £2.5 million figure. It is also understood there is a 15% sell-on clause.
The defender was deemed surplus to requirements at Pride Park after the recent trasfer of Andre Wisdom from Liverpool.
The Coventry City graduate spent five years with The Sky Blues before switching to Derby in July 2014 where he has played 119 games.
The 24 year old has also made 10 appearances for his country, scoring twice.
The signing completes a busy day of business for Middlesbrough FC, after completing the signing of Jonny Howson from Norwich City.