An Irish nanny has been charged with first-degree murder in the US after the death of a child in January.

Authorities say a grand jury in Massachusetts indicted 34-year-old Aisling McCarthy Brady on charges of first-degree murder and assault and battery on a child causing bodily injury.

Ms McCarthy was arrested after the death of Rehma Sabir, who was hospitalised with severe head injuries on her first birthday and died two days later.

Ms McCarthy previously pleaded not guilty to assault and battery on a child causing substantial bodily injury.

She was jailed in lieu of $500,000 in bail while authorities awaited the child's autopsy results.

Her lawyer, who couldn't be immediately contacted in the aftermath of today’s charge, had protested the high bail and said she was "an innocent person sitting in jail".