If the account is on shore and earning interest then the Revenue will in all liklihood have details of the interest earned on the account already. If you have an investigation into your affairs for some other reason then if a statement of assets and liabilities is required you would be required to enter it on that form, so before then you would have to declare any interest earned on it (and pay tax, interest and penalties if applicable).
From 1 March when the new Money Laundering Regulations come into force not reporting the income is treated as a money laundering offence and you could in theory be prosecuted if it came to light.
The Revenue will assume should the account come to light that all the money in it has come from an undeclared source of income unless you can prove otherwise...you should discuss this issue in confidence with a professional adviser immediately.
Ian McTernan CTA
McTernan Associates Ltd
Chartered Tax Advisers
ian@imcternan.com