Used to work there myself. They have internal exams within the Inspector's Offices which are voluntary. Cannot remember what they are called and if they are still being done. However no matter what area of the Revenue you end up in, they do facilitate through Civil Service rules for exams to be taken (AITI, accountancy etc) and for refunds but it depends on if you actually end up in Revenue and not in some other department. (SLight tweaking of rules)
You might try the Dept. of Finances webiste where the give the general rules regarding leave, refunds etc.