Police said the three children, a man and a woman were taken to hospitals around Dublin after the incident occurred shortly after 1.30pm.

One of the children, a girl, and the woman are being treated for serious injuries.

The other two children and the man are being treated for less serious injuries, according to police.

The scene at Parnell Square East remains sealed off and police are being assisted by other emergency services.

Police said they are following a "definite line of inquiry" and are not looking for any other person at this time.

Stephen Murphy, Sky News' Ireland correspondent, said witnesses have told police they saw a man who was reportedly wielding a knife, "attack several young people" as they left the school.

The school is believed to be a largely Irish speaking institution and caters to primary school aged pupils.

"It appears that bystanders and eyewitnesses have played a huge role in bringing this incident to a halt," Murphy said.

"Witnesses have spoken of seeing several bystanders become involved to try and intervene and restrain this man."

He described the area of Parnell Square as an extremely busy area "right in the heart of Dublin city".

"Five casualties have been taken to various hospitals in the Dublin region," an Irish police spokesperson said.

"The casualties include an adult male, an adult female and three young children.

"One child, a girl, has sustained serious injuries, the other two children are being treated for less serious injuries.

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"An Garda Siochana is in contact with parents of all three injured children."

Helen McEntee, Irish justice minister, said she was "deeply shocked" by the "appalling attack on three innocent children and a woman".

"All our thoughts are with those injured, especially with the children, their parents and families, during this extremely difficult period," she said.