The Official Online RBS 6 Nations Store is open. The store has everything you need to get behind your team during the RBS 6 Nations, plus the store is now fully stocked with a much wider range of rugby merchandise.
The 22-year-old will team up with Stephen Ferris and Jamie Heaslip in the Ireland back row with Shane Jennings drafted in as a replacement for the match at Lansdowne Road.
"Sean has a skin infection on his foot which has failed to recover as quickly as expected," read an Irish Rugby Football Union statement.
O'Brien's withdrawal follows hot on the heels of the withdrawal of captain Paul O'Connell and scrum-half Conor Murray, both of whom have been ruled out of the remainder of the Championship as Ireland count the cost of playing four matches in four weeks.
Donncha Ryan has been selected to take O'Connell's place while Eoin Reddan will wear the No.9 shirt against Scotland. Hooker Rory Best has assumed the captaincy.