Non-disclosure of rental income

A new client has undisclosed rental income going back to 1991. During that time the tenants have mainly been family members paying an amount of rent that barely covered the mortgage and expenses, so the amount of tax due is likely to be fairly low. The client's mother occupies part of the property, this being the reason for the original purchase.

Unfortunately this is not a voluntary disclosure, the client has received a letter from the Revenue out of the blue.

Will the client be required to complete tax returns for all these years, or will it be a matter of drawing up income and expenditure accounts for this source and only submitting these?

Any advice would be gratefully received.
Anonymous