Officers were awaiting the results of a postmortem on the baby due to take place today.
Garda sources said the girl was dead when they arrived at the house in Dublin’s north inner city. The infant was removed by paramedics to nearby Temple Street Hospital.
Gardaí were alerted by someone in the house, on Sherrard Avenue, just off North Circular Road, at around 7.30am, arriving shortly afterwards.
There were reports that there might have been blood around the mouth of the child.
Garda sources said there seemed to be some suspicious circumstances to the case, but stressed it was too early to say and that they had to wait for the pathologist’s report.
The mother, aged 42, was in the house at the time, along with three other children, aged nine, 10 and 11. The father is working in Liverpool and is understood to have been on his way home.
Gardaí are handling the matter sensitively, conscious of the remaining siblings’ ages and the trauma the family is suffering.
Gardaí at Mountjoy Garda Station are investigating.