The 21-year-old, who is the son of the late Finance Minister Brian Lenihan, said he suffered a panic attack before the test two weeks ago.
College authorities are allowing the Trinity student to repeat the exam and no further disciplinary action is to be taken against him.
In a statement to the Irish Independent, Lenihan said: “I have been diagnosed with depression, and unfortunately, I had neglected my medical treatment, which exacerbated my condition.”
Lenihan, a business and law student, said he was “unable to cope mentally and physically” and that “getting through the day became a struggle”.
“As the exams loomed closer,” he said, “I became more stressed. On the morning of the exam in question, I had a panic attack and in a random irrational reaction, I took [a] note into the exam hall. I had suffered several panic attacks throughout the exam period.”
The Irish Independent reports that he was elected students’ union president last February after running a campaign which highlighted student mental health as a key issue.
Despite the controversy, he still intends to take up the presidency when his term begins next month.