Thursday’s incident, which police said is not believed to be terrorism-related, saw a man armed with a knife attack the victims outside the school, according to media and witnesses.
Witnesses described how one man had been disarmed, and Prime Minister Leo Varadkar said a suspect had been arrested.
Superintendent Liam Geraghty later told the media that “a young five-year-old girl has suffered serious injuries” and is receiving emergency medical treatment.
He said a five-year-old boy and a six-year-old girl suffered less serious injuries and the boy was released.
The woman was being treated in hospital for serious injuries, while Geraghty revealed that the man, described as being in his 50s, was a “person of interest” to police.
Siobhan Kearney said the scene was “absolutely disgusting” as she initially watched the events from across the street.
“Without thinking, I crossed the road to help,” she told Irish national broadcaster RTE.
“We found another young man who was unarmed (the attacker) with a knife. Another person took the knife and put it away for (police) to find.”