Its variant Merryn was the name of a Cornish saint.
In the English-speaking world, all forms have been occasionally used from the 19th century onwards.
Magenta was a character in “The Rocky Horror Picture Show” musical play and movie. She was a domestic maid played by Patricia Quinn.
This spelling was used by Lord Byron for characters in The Giaour (1813) and Don Juan (1819), and it is through him that the name was introduced to the English-speaking world.