Ceirdwyn was born a Celt in Britannia in AD 29.
She became a warrior in her tribe and fought against the Romans. In AD 60, she was killed while fighting the Romans in a small skirmish, and revived as an Immortal. She soon met her first teacher, Roman General Marcus Constantine, who taught her about her immortality.
Marcus fell for the beautiful warrior woman, and convinced her to come with him to Rome. The two lived together until AD 73, when Ceirdwyn decided to follow her own path. She later returned to her native Britain and met her first pupil, Alex Raven, in AD 75.
In 1255 she met her second student, Matthew of Salisbury, also known as Matthew McCormick.In 1746, Ceirdwyn, under the alias Flora McDonald was in Scotland. While there she met Prince Charles Edward Stewart, who was in the company of Duncan MacLeod. After seeing the Prince to freedom, MacLeod went on a rampage of his own against the English, until Ceirdwyn managed to talk him out of his blood lust.
In the 1980s, she met a mortal man named Steven Jarmel, with whom she fell in love. She told him about her immortality, and the two married in 1983. In 1995, after leaving a restaurant, Ceirdwyn and her husband were gunned down by muggers. After burying her husband, she donned her Celtic warpaint, and carrying her Celtic sword, she went after the men who had killed Steven.
Ceirdwyn killed the trigger man first and went after the rest. Duncan MacLeod, however, managed to find her and convince her to let go of her blood lust just as she told him for do many years ago.She reluctantly agreed, but the terrified thugs planned to kill her before she could kill them. They broke into her house, but she and MacLeod fought them off without fatalities, and they wound up in the hands of the police.