Born in Galway in 1977, Mary credits her sense of wonder and love of nature to her childhood in rural county Galway. The constant change of light and weather influence her distinctly Irish landscape paintings. She studied art in the Limerick School of Art and Design where she achieved a First Class Honours BA Degree in Fine Art Painting  in 2001.  She went on to gain a First Class Honours Higher Diploma in Art and Design Teacher Education in LSAD in 2003. In 1998, she completed an Erasmus student exchange with Athens School of Fine Art. She painted on various Greek islands and studied the traditional skill of icon painting.  

She was awarded the Revenue Commissioner's Bursary Purchase Prize for her Degree Show in 2000. Fahy's work was selected for the 'Young Contemporaries' exhibition in the Belltable, Limerick in 2000. She was twice short-listed for the Countess Markievicz Medal for Painting by the United Artists Club in Dublin. She won both the Larkin Memorial Award and the Irish Times Award for achievements in art teaching in 2003.  Fahy has exhibited a number of educational projects in the National Museum of Ireland and in IMMA, the Irish Museum of Modern Art. 

She holds a higher diploma in French from NUIG. She was Artist in Residence in Camac Arts Centre, France, in 2012.  She has also been granted a residency in the Tyrone Guthrie Centre.

Moving to North Clare in 2003 to work full time as an art educator had a huge impact on her art, with the Wild Atlantic Way providing ready inspiration for seascapes and hiking in the Burren offering imagery for her landscape paintings. Mary often works from her home studio at the kitchen table in Ennistymon, yet she prefers to paint outdoors, directly onto canvas, to capture the immediacy of the moment. Her paintings begin with direct observation of her surroundings. Working quickly, in a gestural way, she allows herself to be lead by the process. 

Fahy's work is in numerous public and private collections, both national and international, including the Revenue Commissioners, The Boyle Civic Art Collection, Camac Arts Centre in France, and in the collection of President Michael D. Higgins. Her work is held in private collections worldwide, including Ireland, the UK, Luxemburg, Switzerland, Belgium, Italy, France, Germany, The USA, Australia and The UAE.