Five people, including three children, were taken to hospital on Thursday, Irish police said, after media reports of a stabbing outside a central Dublin school.
Police declared a major incident and threw up a cordon around Parnell Square in the Irish capital but do not suspect a terror motive.
Irish media outlets reported the incident was suspected to be a stabbing.
Prime Minister Leo Varadkar said a suspect had been arrested while the police said they were "following a definite line of inquiry."
"An Garda Siochana is not looking for any other person at this time," the force said in a statement.
It added: "Five casualties have been taken to various hospitals in the Dublin Region.
"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.
"The adult female is being treated for serious injuries and the adult male for less serious injuries."
The statement added that officers were "investigating all the circumstances" of what it called a "serious public order incident" which occurred shortly after 1:30 pm (1330 GMT).
It said the force was in contact with parents of all three injured children.
Varadkar said he was shocked by the incident, which local media said took place outside a primary school.
"The emergency services responded very quickly and were on site within minutes. I thank them for that," he said in a statement.
"Gardai have detained a suspect and are following a definite line of inquiry."
Local lawmaker Aodhan O Riordai said the reports of a "stabbing incident" were "disturbing."
"Understand an individual has been detained. Hope injuries are not serious but it will extremely traumatising regardless for all involved," he wrote on X, formerly Twitter. (AFP)