Irish Rail was ordered in January to drop a TV ad offering inter-city journeys for €9.99.
The Advertising Standards Authority says it is impossible for customers booking their tickets online to get their ticket for under €10.
The ruling comes after one customer complained that their €9.99 fare actually cost nearly €13 when credit card and booking fees were included.
After the ASAI ordered Irish Rail not to run the ad again, Irish Rail broadcast a modified version for the last week of the campaign in January, which it says meets the ASAI code.