I think you'll find it difficult to get anywhere regarding the facts of the matter. However, you might get somewhere with mitigation.
Apply for a Student Travelcard as soon as possible and keep the ticket he uses for the return leg. Write a letter, enclosing copies of them, stating it wasn't his fault* (Do I have this correct, that you bought the ticket for him?) and see if they will the agree to just charging the fine or the full fare, not both. Follow up two days later with a phone call. Be conciliatory (but not grovelling) and ask if some consideration can be made. Be willing to pay whatever is agreed.
http://www.studenttravelcard.ie/index.aspx
* As there are bye-laws in place and as he is the one who travelled without a proper ticket, any court appearance would be in his name.