Marsh met Mary Swanzy at May Manning's art classes, with Swanzy remembering Marsh as being from "a background of impecuniosity, which did not apparently worry them in spite of a more affluent upbringing". Marsh was influenced artistically by her aunt and John Butler Yeats, with whom she became close friends. In summer of 1898, Yeats painted Marsh's portrait at Manning's studio. Marsh was more drawn to the work of Yeats than of his son, Jack, and modelled her portraits on that of the older Yeats. He mentored her, encouraging her to see other artists' work as much as possible and saying "to produce a picture will force you to think." He urged her to paint more industriously. She exhibited with the Royal Hibernian Academy (RHA) for the first time in 1900 with East wind effect and Roses. Yeats later claimed that Marsh helped him with "line drawing or sketching, by putting him on the track of bulk drawing."[1][2]
Alongside Manning's classes, she took night classes in sculpture with John Hughes and Oliver Sheppard at the Metropolitan School of Art. Aside from a trip to Paris in 1910 or 1911, Marsh seems to have been taught exclusively in Dublin throughout her 20s. She took a course at Norman Garstin's studio in Penzance, and stayed in North Wales in 1914, painting two Trearddur Bay scenes. Marsh painted still life and portraits, including one of Lily Yeats. It appears that her portraits of children and dogs were popular based on her submitted works to the RHA, exhibiting without a break from 1900 to 1921. The Hugh Lane Gallery holds her portrait of Lord Ashbourne, which demonstrates her painting style of loose brush strokes with an air of informality. Yeats suggested that she spend some time in the United States, where he was living at the time. She spent two months in New York, staying with cousins at White Plains and then moved into a room neighbouring that of Yeats in Petitpas. Her uncle strongly disproved of this living arrangement, so she left and returned to Ireland in January 1912, which upset Yeats greatly.[1][2][3]
Upon her return from New York, Marsh started holding classes at her studio at South Anne Street which Swanzy recalled were "well liked and always full", with Mrs Susan Yeats becoming a pupil. Marsh became the Professor of Fine Arts at Alexandra College in 1916. In the same year, Marsh painted the fires and destruction of the 1916 Easter Rising. She painted a portrait of Jack Butler Yeats in 1918, which is now held by the Highlanes Gallery. John Butler Yeats later sympathised with her in a letter that she and other women were not elected members of the RHA. Knowing that Yeats was in financial difficulty, she sold some of his drawings and sent the money to him. It appears that over time, Marsh worked more with colour, as demonstrated in her portrait of Mrs Susan Yeats. Her final paintings were night studies, some of which show a possible influence from Swanzy with whom she shared a studio in autumn 1920. She is also believed to have been one of the founding members of the Society of Dublin Painters.[1][4][5]
