Meaning: of unknown meaning though it could be linked to Old Irish midithir meaning "to measure" or "to judge".
In Irish mythology. Midir is the son of the Dagda. He is well-known for falling in love with Etain despite already being married. When his wife Fuamnach found out, she was furious and used magic to turn her into a pool of water then conjured up a magic wind to dry her up; however, the steam from the water condensed into a butterfly. But Midir recognized Etain as the butterfly because of the music that beat from her wings and her beautiful scent and wherever he went, Etain was always with him.
So Fuamnach conjured up a magical storm that dragged Etain from Midir, lasting for years until it wore out and she found herself near the home of Aengus Og (the Irish god of love and Midir's foster son). Knowing who she was, he built her a little glass home and Etain lived comfortably there for some time until she flew out of the house and was swept away yet again by the magic storm, stuck within it for 7 years until it blew her through the window of a human king's hall, where she fell into the queen's wine goblet and was drunk up. The queen later gave birth to a beautiful baby girl whom they named Etain. She grew up to be a very beautiful woman with no memory of her previous life and later married a king. However, Midir found her and told her about their previous life together and how he had been searching for her for 300 years, but she didn't believe him and told him she was a married woman. When he asked her if she would come away with him if he got her husband's permission Etain agreed and Midir set up a plan by playing chess with the king and letting him win game after game until, cocky in his victory, the king wagered that the victor of the next game could ask for anything he wanted from the loser. That's when Midir won the next game and asked to kiss the king's wife, Etain.
Unwilling to do so, the king asked for a month's grace and Midir agreed, during which the king prepared an army to prevent him from going near Etain. But Midir used his magic, embraced Etain, and when he did so she remembered everything from her previous life, and became an immortal once again, disappearing with Midir.