If you have missed the date to defer/cancel your exam (6 weeks from the trimester start date), then you will need to re-register for the module when you wish to complete the exam. The re-registration fee is €180.
Be aware that with the re-registration, it will cap the results at a pass mark which is a D- Grade
Alternatively, depending on the reasons, you may be eligible to apply for extenuating circumstances. You can view the extenuating circumstances policy and application form in the 'After the exams' section here: https://iob.ie/info/student-info#exams-and-regulations