On 7 August 1944, McNicholas survived the sinking of the Amsterdam; a hospital carrier transporting casualties to Britain from Normandy, France.

[7] The Amsterdam was on her third cross-Channel voyage to pick up Allied casualties and German prisoners of war from the Battle of the Falaise Pocket, when disaster occurred.

An official record states that the sinking was the result of "enemy action,"[9] and it was widely reported in the press that the ship was hit by a torpedo.

On the arrival of an American cutter, McNicholas was rescued from the sea and continued to provide medical aid to the injured men as they were pulled from the water.

"[12] According to a magazine article written in 2013, McNicholas declined to attend her investiture at Buckingham Palace, travelling instead to visit the bereaved parents of her best friend.

'Sister Lily McNicholas' is featured in the Our Irish Women temporary exhibition, on display at the National Museum of Ireland - Country Life, during October and November 2018.