I believe she was killed by her mentally ill father.
The following excerpt is from the Catholic Encyclopedia. Please note that it is a legend, and identical with many pagan legends also found at the time. So we probably do not know the real story, only that she did exist and that many miracles have been attributed to her.
According to the narrative, Dymphna - the daughter of a pagan king of Ireland - became a Christian and was secretly baptized. After the death of her mother, who was of extraordinary beauty, her father desired to marry his own daughter, who was just as beautiful, but she fled with the priest Gerebernus and landed at Antwerp. Thence they went to the village of Gheel, where there was a chapel of St. Martin, beside which they took up their abode. The messengers of her father however, discovered their whereabouts; the father betook himself thither and renewed his offer. Seeing that all was in vain, he commanded his servants to slay the priest, while he himself struck off the head of his daughter. The corpses were put in sarcophagi and entombed in a cave where they were found later. The body of St. Dymphna was buried in the church of Gheel, and the bones of St. Gerebernus were transferred to Xanten
Tradition tells us that her father, Damon, beheaded her for refusing to leave Belgium and return to Ireland to become his wife. She was 15 years of age at the time.
Dymphna died in 6##.
Dymphna Clark died in 2000.
St Dymphna
God has never shown Himself to any person, for if He did, they would die from His glory. He has appeared as Jesus, and has sent others to reveal His message. St. Dymphna, though, was not said to have had visions of Christ or others.
.Catholic AnswerThe legend of St. Dymphna follows the same course as other stories about a King who decides to marry his own daughter. In this particular case, she refused, and the King himself cut off her head. But it is just a legend, and there is no corroborative evidence that she ever existed nor that she is St. Damhnat of Ireland. Note, St. Dymphna definitely existed and we have her relics, but the stories about her cannot be substantiated. Short answer? She was beheaded by her father, the King in the 7th century. The complete story of St. Dymphna can be found at the second link below:
St. Dymphna was killed in the year 650, and was only around 15 years old.
St. Bartholomew the Apostle and St. Dymphna
St. Dymphna, the patron saint of those suffering from some form of mental illness, is memorialized on May 15.
Dymphna Cusack died on October 19, 1981, in Manly, Sydney, New South Wales, Australia.
.Catholic AnswerThe legend of Saint Dymphna, a daughter of an ancient King in Ireland, has no basis in any history outside of itself, and there is no proof that St. Dymphna is the same person as St. Damhnat of Ireland. All we have is the legend, which certainly does not follow the custom of modern times of everyone having a "last name".
St. Dymphna is famous for leaving Ireland, with her priest, in order to escape the incestuous advances of her father. When her father finally caught up with them, he killed the priest & then tried to make Dymphna go back to Ireland with him to be his wife. When she refused, he killed her too.
There isn't a specific patron saint of psychiatrists recognized by the Catholic Church. However, some may turn to St. Dymphna, who is often invoked for mental health issues and emotional distress.