Costelow, 25, suffered an ankle injury in the second half of the 26‑23 defeat by Scotland and will not travel to Dublin. He may still be available for the final game against Italy on 14 March, but the squad will need to rely on Jarrod Evans or Dan Edwards to take the number 10 jersey.
Wales also lose flanker Taine Plumtree to a shoulder injury, but coach Steve Tandy has added prop Keiron Assiratti and Dragons flanker Ryan Woodman to the squad. With 24 losses in 26 internationals, the team is desperate to end a 14‑match losing streak and has not beaten Ireland away since 2012 or won in Dublin since 2015.