However, authorities in the US say the charge is expected to be upgraded to murder when the state medical examiner completes the autopsy.
Ms McCarthy Brady, who cared for little Rehma Sabir for six months, denies causing devastating injuries to the baby at her home in Boston.
The nanny -- who has lived illegally in the US for 10 years -- was in sole charge of the child on January 14 when she called 911 for help.
The 34-year-old, from Moher, Co Cavan, appeared in court yesterday charged with assault and battery. Bail was set at $500,000.
The Irish woman’s defence lawyer said: “There’s more to the story than you’ve heard.”
Rehma had a broken leg and arm and was bleeding severely from her head and eyes when police found her on January 14.
She later died in hospital from serious swelling after doctors took her off life-support.
Ms McCarthy Brady remains in custody today after bail was set at $500,000 after she pleaded not guilty to a charge of assault and.
The Cavan native hid behind a wall as she pleaded not guilty at her arraignment in Cambridge District Court.
Prosecutor Katharine Folger told the court that she expects the charges to be upgraded to homicide.
Brady's attorney, Melinda Thompson, said her client has been a nanny for 13 years and never had any incidents. They claim the child has travelled a great deal and that “anything could've happened” during those visits overseas.
Middlesex District Attorney Gerry Leone later said: “This is an extremely troubling case, where we allege the defendant violently assaulted a one-year-old child, causing a devastating head injury and broken bones.
“Children are our most vulnerable victims and where, as here, the offender has been entrusted with the care of a child who depends on them, the allegations are all the more egregious.''
A statement on the District Attorney's website said: “It is alleged that on January 14, the child was in the care of the defendant, her nanny.
“Through their investigation, including interviews with witnesses, police determined that the defendant had sole custody of and contact with the child during the time that she sustained injuries consistent with abusive head trauma.''
Rehma's injuries could have happened at any time, Brady's defence lawyer said.
Brady is originally from the Moher area in County Cavan, where her mother Margaret and other family still live.
She left Ireland for Massachusetts 10 years ago chasing the American dream.
She entered the US in August 2002 on a 90-day visa – but US authorities say she never left and has been residing there as an illegal immigrant.
Neighbours say the Irish woman appeared to be a caring and kind nanny. She worked with children for 13 years and was often seen walking around the neighbourhood – with some locals observing how she often would wave and say ‘hello'.
Ms McCarthy married Cork painter Donald McCarthy, in Boston in September.
But a picture has now emerged of a woman that had a number of run-ins with the law.
The 34-year-old had two restraining orders issued against her – for allegations relating to harassment and assault.
Just last March, a woman took out a restraining order against her, claiming that she “texted me and accused me of taking her student that was arriving from Ireland in a week.”
Ms McCarthy was accused of setting up a fake Facebook page in the woman's name before harassing her boss.
She was also accused of posting comments on a parenting website in which she alleged seeing the woman “abusing” kids in a playground.
Just five years previously, the Irish woman had another scrape with the law.
She was accused of getting into a fight involving a roommate but assault charges were later dropped.
And in 2005, her former boyfriend David Furey took a restraining order against her.
In court papers at the time, he claimed that she attacked him in a bar room, threw a beer bottle at one of his female friends before threatening to make his life a “living hell”.
But speaking after the charges against her over Rehma’s death, Mr Furey said he believes she is innocent.
“She always took care of kids. She was always good with kids. I was very shocked to hear that on the news today. I do believe that she's innocent.”
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