The State Examinations Commission has confirmed that it is investigating a complaint that a teacher was spotted correcting a Leaving Certificate paper on a DART.
It is reported that the teacher in question was marking a higher level history paper on the busy train.
A spokesperson for the State exam body said: "Exam papers should not be corrected in public, but rather in a secure location and where a breach comes to light, it must be investigated."
In a statement, the commission said: "The State Examinations Commission cannot comment on individual cases.
"However, it can confirm that it is currently investigating a report of a possible breach of protocol for the marking of examination material."
It said: "All examiners are required to uphold the fundamental principles regarding the marking process which include the safeguarding of candidate confidentiality."