Olympic and world champion Femke Bol was one of many people to post messages of support for Mageean.
The Dutch runner said she was sending her “love and strength”.
Mageean’s Irish team-mate Sarah Lavin said: “I don’t have the words to send you. You are so loved and so strong and I wish I could give you the biggest hug.”
Ireland’s double Olympic boxing champion Kellie Harrington said “we all love you and will be praying for you and keeping positive spirits up for you”.
European Athletics posted on X: “We’re all behind you, Ciara. Everyone at European Athletics sends their best wishes to Ciara Mageean following her recent cancer diagnosis.”
Portaferry GAA, who Mageean used to represent in camogie, said she is “truly an inspiration to so many people”.
“We are all behind you in this fight,” the club said.
“You and your family as always have our full support every step of the way.
“We know you’ll be back better and strong. We send our love and best wishes to you Ciara.”