Jonathan Sexton has passed two levels of head injury assessment and will undergo a further test on Tuesday after he was involved in a clash of heads last weekend.
Ireland fly-half Sexton, who was forced spend 12 weeks on the sidelines after suffering four concussions in 2014, was substituted following a collision with Brendan Macken during Leinster's 51-10 defeat to Wasps in the European Rugby Champions Cup on Saturday.
But Sexton appears to be making progress in recovering from that blow as he bids to be available for Ireland's Six Nations opener with Wales on February 7.
A squad update from the Irish Rugby Football Union read (IRFU): "Johnny Sexton has passed HIA 1, HIA 2 and will complete HIA 3 tomorrow [Tuesday]."
Meanwhile, the IRFU also confirmed Connacht prop Finlay Bealham has replaced Marty Moore in the squad for the Six Nations.
Leinster's Moore has been ruled out of the championship with a hamstring injury sustained in the loss at Wasps, who announced on Monday the tighthead would join the Premiership club for the start of the 2016-17 season.
Connacht's Robbie Henshaw suffered no ill-effects following his return to action from a broken hand that had kept him out since November.