She was an Irish princess whose father (apparently unhinged by grief over the death of his wife-her mother- whom Dymphna apparently bore a striking resemblance to) was told by his advisors to marry.
She rejected his advances, fleeing Ireland along with her confessor, St.Gerebran, to what is now Belgium. Her demented father pursued her there, and after Gerebran tried to dissuade him from his unnatural desire for his daughter, summarily beheaded the priest. Dymphna refused his advances and was likewise dealt with.
The site of her martyrdom is Gheel, Belgium which is a place of pilgrimage from all suffering from mental or emotional disorders and her feast day is May 15.