Life at Leamaneh Castle
Seven months after Neylon's death, she married her cousin, Conor O'Brien of Leamaneh. The couple extended the O'Brien tower house at Leamaneh, creating Leamaneh Castle. An inscription on the castle reads: "This was built in the year of Our Lord 1648 by Conor O'Brien and Mary ní Mahon alias Brien wife to the said Conor". The couple had eight children, the eldest of whom, Sir Donough O'Brien, was born in 1642. 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).
It is with Leamaneh Castle that some of the stories of O'Brien's claimed violent actions are associated. It is alleged she would hang servants who displeased her, that she victimised trespassers, and denied rights of way through her land.
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. O'Brien is reported to have ridden with her husband during some raids. In depositions relating to the Irish Rebellion of 1641, a man named Gregory Hickman stated that: "Conor O’Brien, gentleman, in a most rebellious manner seized upon the deponent's corn"; and, later, "Conor O’Brien, of Lemaneagh, accompanied by Mary Brien [and others] with force of arms came to the deponent's house and took away fourteen English swine and a parcel of household stuff; also 400 sheep". Some of those raided by O'Brien reputedly perpetuated rumours that she was a witch.
Conor O'Brien was commissioned as Colonel of Horse in 1650 in the army of Charles II. He died the "death of a hero" in 1651 from wounds he received fighting against Edmund Ludlow at the pass of Inchecrogan. Some stories suggest that he was returned to Leamaneh by O'Brien's own troops, gravely wounded, and that she nursed her husband on his death bed. 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. 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. Travelling through all the devastation and sorrow of his death, she arrived at the Limerick camp of Henry Ireton. Here, dressed in blue silk and lace, she reportedly proclaimed that she was willing to marry any of the officers. Some sources suggest that she made this offer as proof of her husband's death. In other sources it is suggested that she may have wanted to marry a Cromwellian officer in an effort of secure her property and land from seizure.
Some legends, described by antiquarian Thomas Johnson Westropp as "less credible" and an attempt to present O'Brien as a type of "female Blue Beard", propose that O'Brien married between 12 and 25 Cromwellian officers. Folklorist sources suggest that these numbers relate to love affairs had by O'Brien. These apocryphal stories claim that after every marriage or love affair, she killed each suitor, including killing one with a kick to the stomach. 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. 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.
According to historical texts, soon after her husband's death in battle, O'Brien petitioned Charles I's Lord Deputy in Ireland, Ulick Burke, to become the custodian of the O'Brien estate. She was able to secure the land for her children, but she could not retain Leamaneh Castle, which fell to the Cromwellian army and was turned into a garrison.