She was born at Portmarnock, County Dublin, the second daughter of historian and political figure Eoin MacNeill and Agnes Moore.
[1] In 1935, Séamus Ó Duilearga invited her to work for the newly founded Irish Folklore Commission as office manager.
She trained in folklore methods at Uppsala University, Sweden, before starting research on the Lughnasadh festival.
[1] MacNeill left the Folklore Commission in 1949 to go to Boston where she married poet John L. Sweeney.
[1] She bequeathed two exceptional cubist paintings by Picasso and Juan Gris at the National Gallery of Ireland (Dublin).