Despite a long career, Saint-Ruhe is remembered largely for his brief service in Ireland during the Williamite War, in which France provided military support to the Jacobite forces of James II.

Alexander Franklin of St. Mark's Church, Dublin, claimed to have met a great-grandson of Saint-Ruhe serving in the Fitzjames regiment of the Irish Brigade around the time of the French Revolution.

Louis treated her with great sympathy and ordered her husband to stop, but when the ill-treatment continued the King began sending him on unnecessary missions to free his wife of his company.

[14] With a temporary general's commission and accompanied by lieutenant-generals de Tessé and d'Usson, Saint-Ruhe arrived at Limerick on 9 May 1691, bringing sufficient arms, corn and meal to sustain the army until the autumn.

[16] However, by late spring 1691, Ginkel was concerned that a French convoy could land further reinforcements at Galway or Limerick, and began planning to enter the field as quickly as possible.

Saint-Ruhe had been unsure where Ginkel would attempt to cross the Shannon, but by 19 June he realised Athlone was the target and began concentrating his troops west of the town.

[18] Ginkel breached the Jacobite lines of defence and took Athlone on 30 June after a bloody siege; Saint-Ruhe was unable to relieve the town and fell back to the west, his army depleted by large-scale desertions.

[19] He reinforced the morale of the rank and file by stressing that the conflict was a defense of the Catholic Church in Ireland against religious persecution and by "creating a personality cult around himself with bloodthirsty promises of what the army would do to its foes".

Saint-Ruhe had chosen an extremely strong defensive position, his infantry occupying the slopes of a ridge known as Kilcommadan Hill, with its flanks protected by marshy ground.

[19] According to the Jacobite author Nicholas Plunkett, Saint-Ruhe's body was carried off and brought to the town of Loughrea, where it was later interred privately at night at the Carmelite Abbey cemetery.

[7] At the spot where Saint-Ruhe supposedly fell a whitethorn grew, afterwards named "St Ruth's Bush"; a light was said to have been seen dancing around it at night, while visitors took away twigs from it as souvenirs.