Based on what the OP has said, the suggestion from this end would be to:
1) Ignore the debt collector.
2) Use online banking to pay €50 a month directly into the College's bank account, quoting your reference number, which is usually your student number.
Chances are that the College will leave you alone while you are doing this. After 32 months (2 years, eight months) you will have settled your account in full. In the unlikely event that they catch up with you in the mean-time and manage to server a court summons, go along to the court and let the judge know what happened, that you are paying €50 a month, and that there is no need for a judgement because you are in the middle of paying your debt in full.