Ulster flanker Jordi Murphy has been withdrawn from the squad to face the Ospreys in Belfast tonight and looks likely to replace the injured Jack Conan in Ireland's World Cup squad.
Murphy had starred for Ireland and Leinster in 2018 but moved north last season to get more regular game time, with Sean O'Brien and Dan Leavy ahead of him in the queue.
Prior to that he was also in competition with Jamie Heaslip for a back row place and while Devin Toner's omission from Ireland's World Cup squad made the most headlines, Murphy had grounds to feel hard done by when Conan, Rhys Ruddock and Tadhg Beirne moved ahead in the pecking order.
Conan, who impressed off the bench in the 27-3 win against Scotland last Sunday, sustained a foot injury in training.
The setback means he will miss the clash with Japan on Saturday, with O’Mahony retaining his place in the starting line-up.
Murphy, who had already been on standby for Ireland action, has been replaced in the Ulster team by Sean Reidy, with Clive Ross drafted on to the bench.
Follow Ireland v Japan on Saturday (kick-off 8.15am) via the live blog on RTÉ.ie/Sport andthe NewsNowApp, watch live on RTÉ2 from 7.30am or listen to live match commentary on RTÉ Radio 1.