The woman, aged 52, entered the all-boys secondary school on the banks of the River Moy in Ballina, wielding an umbrella and a large knife.
Local gardaí and an armed response unit were dispatched to the school after receiving reports of a woman in possession of a knife.
According to a witness, the woman approached the staff member wielding an umbrella and a large knife. She then proceeded to strike him with the umbrella but did not use the knife.
The woman was arrested and taken to Ballina Garda Station. No one was harmed during the incident.
The woman, who is not thought to be a parent of a student, was later released without charge.
A tweet was issued by the college after the incident to reassure students and parents. It read: “The school management of St Muredach’s College can confirm that an unauthorised person was removed from the school by An Garda Siochána.
“At no time was there any risk to the students attending the school.”