Legends of the Tuatha de Danaan: Second Battle of Magh Tuireadh
pre-12th century—present. Old Irish | Modern Irish, folklore.
Men that were wounded to death in the battle would be brought to the spring and put into it as dead men, and they would come out of it alive and well and fitter even than they had been before.
During an invasion of the Tuatha de Danaan's Ireland by the Fomorians, Diancecht the physician promises for his part, before the Second Battle of Magh Tuireadh, that he will heal everyone who suffers a wound, unless he has lost his head or had his brains sliced through or his back cut in two.
But then he seems to get even better at his job than this. On the Fomorian side, if men were killed one day they remained dead the next morning, but not so for the Tuatha de Danaan; a man of their's killed one day would be revived and fit to fight again the next. For a spring lay to the west of the battlefield and Diancecht and his son and daughter sang incantations over the water and threw herbs into it –
...and the men that were wounded to death in the battle would be brought to the well and put into it as dead men, and they would come out of it whole and sound, through the power of the spells.