Seven months after Neylon's death, she married her cousin, Conor O'Brien of Leamaneh.
They 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 her servants who displeased her from the corbels of the castle.
It is also said 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".

Conor O'Brien was commissioned as Colonel of Horse in 1650 in the army of Charles II, dying in 1651 from wounds he received fighting against Edmund Ludlow at the pass of Inchecrogan.
It is told that when his body was returned to Leamaneh, O'Brien leaned out of a window and declared that "We want no dead men here".
Other accounts state that when the soldiers explained he wasn't dead yet, she let them in and nursed her husband on his death bed.
Lady Chatterton's account in 1839 says that during the battle of the pass of Inchicronan, Ireton sent five of his best men, disguised as sportsmen, 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 Parliament, 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.
Another local folklorist source, from the 19th century, suggests that these numbers relate to secret love affairs O'Brien had with other officers, after she married a Cromwellian officer to save her lands.
These apocryphal stories claim that after every marriage or love affair, she killed each officer in ways that looked accidental.
These methods included pushing them off the top of the castle, suffocating them in bed, pushing them out the window, kicking them in the stomach while shaving, or challenging them to ride to the Cliffs of Moher on her wild blind white stallion - which had been trained to fling the victim off the cliff edge.
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 English 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 th
Seven months after Neylon's death, she married her cousin, Conor O'Brien of Leamaneh. They 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 her servants who displeased her from the corbels of the castle. It is also said 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".
Conor O'Brien was commissioned as Colonel of Horse in 1650 in the army of Charles II, dying in 1651 from wounds he received fighting against Edmund Ludlow at the pass of Inchecrogan. It is told that when his body was returned to Leamaneh, O'Brien leaned out of a window and declared that "We want no dead men here". Other accounts state that when the soldiers explained he wasn't dead yet, she let them in and nursed her husband on his death bed. Lady Chatterton's account in 1839 says that during the battle of the pass of Inchicronan, Ireton sent five of his best men, disguised as sportsmen, 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 Parliament, 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. Another local folklorist source, from the 19th century, suggests that these numbers relate to secret love affairs O'Brien had with other officers, after she married a Cromwellian officer to save her lands. These apocryphal stories claim that after every marriage or love affair, she killed each officer in ways that looked accidental. These methods included pushing them off the top of the castle, suffocating them in bed, pushing them out the window, kicking them in the stomach while shaving, or challenging them to ride to the Cliffs of Moher on her wild blind white stallion - which had been trained to fling the victim off the cliff edge. 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 English 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 th
Máire Rua O'Brien