Joan made the decision after being diagnosed with the muscle-wasting condition, motor neurone disease, last year.
The veteran actress, now in her 60s, said: "I loved the character of Mags, foil to Charlie's shenanigans and I particularly enjoyed her strong relationships with, for example Dolores, Rita, Dermot and Jo. I really enjoyed playing her."
Wise, warm and sensitive, Mags is confidante to all in Carrigstown.
Motor neurone disease is a degenerative condition which destroys the cells that control voluntary muscles and affects speaking, walking, breathing, swallowing and general movement.
Joan's Fair City colleagues yesterday gave her a celebratory send-off and praised her role in the show over the years.
Dave Duffy, who plays taxi boss Leo Dowling, said: "Joan was terrific to work with. Some of the best scenes I ever did were with Joan.
"It is like a small community here and it's always sad when someone leaves who was part of it".
He described how Joan had continued working as long as she could. "We gave her a terrific send-off. We had champagne and flowers for her and a bit of a party in McCoy's pub".