If you have discovered an item, previously submitted, which now requires amending then the best course is usually to draft a letter detailing the amendment and submit it to your district. If this is a self-assessed matter, then an amended notice of assessment will issue. If the amendment results in alteration to liabilities already assessed there may be issues over interest or penalties but that depends on Revenue's view of the nature of the amendment and how it has come to light. Revenue are not generally interested in giving people grief. Believe it or not, all they want is to have a return which fairly reflects the position. If you have an accountant it may be best if they did the submission for you. Remember though, what may not seem big to you may be big to Revenue so I wouldn't recommend just "not bothering".