Remaining on her estate at Leamaneh for several decades, her son Donough O'Brien moved the family seat to the larger Dromoland Castle where she lived until her death in 1686.

[4] The other known children were Teige, Turlough, Murrough, Honora, and Mary, with two other daughters who are believed to have died during the plague which affected the area during the Siege of Limerick (1650–1651).

[6] During the Confederate Wars in Ireland (1641–1653), her husband led and financed one of the five militia companies of Clare which raided tower houses of English settlers planted in the area during the preceding century.

[9] Other stories suggest that his body was returned by Cromwellian troops, and that, in order to avoid confiscation of her lands, she claimed that the dead man was not her husband and that she was already a widow.

[9] Lady Chatterton's account, in 1839, says that during the battle of the pass of Inchicronan, Henry Ireton sent five of his best men to shoot Conor O'Brien, and one of them succeeded in wounding him.

Mary captured and hanged the man, called her sons and advised them to surrender to the Cromwellian forces, and set off in her coach to Limerick.

[12] In one such legend O'Brien challenged a local man, with whom she had a dispute, to ride to the Cliffs of Moher on her wild blind stallion - with the expectation that he would be killed.

The folklorist Máire MacNeill describes parallels between some of these stories, such as challenging suitors to ride a wild stallion, and those of traditional Irish sovereign-goddess myths.

[4] Other stories claim that O'Brien had hung servants, who had displeased her, from the corbels of the castle; the males by the neck and the woman by the hair, and would cut off their breasts.

[2][13] Still an Irish Catholic but attending English Protestant mass, O'Brien reputedly had a dispute with the local rector, which resulted in her building Coad Church.

[14] In 1662, O'Brien was indicted for the murder of a local English landlord, which related to her apparent involvement with her second husband's raiding parties during the early 1640s.

[2] O'Brien is reported to have died an extraordinary death, with one legend stating that she was sealed into a hollow tree in the avenue of Carnelly Forest.

[4] In the Dictionary of Irish Biography, the entry for O'Brien records her death as "conventional", and that "in poor health" in her seventies she made a will.