There are a number of situations where a person exchanging a UK driving licence might not get all the categories on that licence transferred to the Irish licence.
Under EU law, an application for a truck or bus licence (categories C and D) must be accompanied by a completed medical report. If a person with a truck or bus category on their UK licence attends a National Driver Licence Service centre without the relevant medical report, they will be given the option to sign a form stating that they do not wish the categories to be exchanged. The applicant can request the categories be re-instated on their Irish licence by presenting a medical report.
Some UK licence categories, known as national categories, only apply in the UK and are not exchangeable. For example, UK drivers who passed the test for category B (car) before 1 January 1997 were entitled to C1 and C1E (trucks) and D1 and D1E (bus) categories. These categories are given with restrictions identified by codes. The D1 and D1E categories, with a UK restriction code 101, allow the driver to drive a bus "not for hire or reward" in the UK only. As there is no corresponding Irish category, these cannot be exchanged.