Gardaí said the man is receiving medical treatment after the incident at Morning Star national school in Ballyphehane, on the southside of Cork city,
The school has also introduced a new entrance policy for students.
In a statement released yesterday, the school said the man presented at the main school office around 8.45am on Tuesday and claimed he wanted to enrol his son.
They said the man appeared “quite anxious and sounded incoherent” and had no information about a son.
When pressed for information, the school said the man became verbally abusive and was escorted by the principal away from the entrance.
“The man resisted so the principal had to move him away physically,” the school said.
Gardaí were called and other teachers escorted children to class as the principal continued talking to the man, who calmed down. But he left calmly before gardaí arrived.
A Garda spokesman confirmed last night that the man was subsequently arrested and detained under the mental health act, and is receiving treatment.