Irish Independent: Your Questions: What should I do if I failed to declare all my income in tax return?

If you noticed that you have made an error on your tax return, it is advisable to contact Revenue to let it know a self-correction will be made.

According to the chief executive of Taxback.com, Joanna Murphy, Revenue allows a return to be self-corrected without a penalty. You need to notify Revenue in writing or via ROS of the adjustments to be made, and the correct tax and statutory interest payable, provided the correction is made within 12 months of the due date for filing the return.

https://www.independent.ie/business/personal-finance/your-questions-what-should-i-do-if-i-failed-to-declare-all-my-income-in-tax-return-38739851.html