C.S. MacCath was born just a few months before Neil Armstrong took "one small step for man, one giant leap for mankind." She graduated with Distinction from the University of Toronto in 2001 after earning a B.A. in Celtic Studies and graduated with High Distinction from the University of Maine in 2003 after earning a M.A. in English. In 2006, she attended the Clarion Science Fiction and Fantasy Writers' Workshop.

Ceallaigh's fiction and poetry have appeared or are forthcoming in Strange Horizons, Clockwork Phoenix: Tales of Beauty and Strangeness, Murky Depths, Mythic Delirium, Goblin Fruit and others. At present, she is working on the first trilogy of a nine-novel space opera entitled Petals of the Twenty Thousand Blossom and a collection of short stories tentatively entitled Spirit Boat. When she isn't writing, she plays traditional Celtic and West African music.