The toddler is understood to have been stabbed a number of times, including in the chest, at the Riverside apartments, in Kimmage, Dublin 12.
Emergency services were alerted to the scene at around 7pm last night after receiving a number of 999 calls about a woman in a distressed state.
The little boy has been named as Omar Omran. Iraqi doctor Maha Al-Adheem has been named as Omar's mother.
Senior sources last night told the Irish Independent the child was stabbed in the upper-chest with a kitchen knife.
Emergency services personnel had to break down the door in order to get immediate access to the apartment.
When they entered, they discovered the child dead in the bedroom of the property. He was pronounced dead at the scene.
This morning, gardai remained at the scene awaiting the arrival of the pathologist and the garda technical bureau.
One small teddy bear was positioned beside them, and on the gate someone had left a small cuddly giraffe with the message 'Why???? RIP' pinned to it on a sheet of paper.
A resident of Riverside apartments who did not want to be named said he had met the mother in the apartment complex a few times.
"The police and fire brigade were here when I arrived at around seven o'clock, and I was thinking there was a fire," he said.
"People are shocked. She would leave here in the car and the child was always with her. He was always happy, and talked," he said.
At 11.45am the ambulance from the city morgue arrived and the young boy's body was brought out on a stretcher and under a blanket.
The ambulance was then driven from the scene under garda escort and a post mortem was due to place later today.
Local people shocked by the tragedy brought flowers to the scene, pausing briefly to pay their respects.
"We didn't know the child, but it just felt like the right thing to do," said Greg Byrne, who arrived with his friend Reddy Thumma.