As records only have to be kept 6 years it would be pretty hard to prove some costs if asked by revenue 20 years later.
This is an interesting area.
The onus as always is on a person to be able to prepare and file a correct return.
Section 886 TCA 1997 sets out the relevant legislation and it says in the first instance:
"Every person who... is chargeable to capital gains tax in respect of chargeable gains... shall keep, or cause to be kept on that person’s behalf, such records as will enable true returns to be made for the purposes of ... capital gains tax of such chargeable gains."
It then goes on to later specify a retention period as follows:
"Notwithstanding any other law, linking documents and records kept in accordance with
subsections (2) and
(3) shall be retained by the person required to keep the records—
(i) for a period of 6 years after the completion of the transactions, acts or operations to which they relate, or
(ii) in the case of a person who fails to comply with Chapter 3 of Part 41A requiring the preparation and delivery of a return on or before the specified return date for a year of assessment or an accounting period, as the case may be, until the expiry of a period of 6 years from the end of the year of assessment or accounting period, as the case may be, in which a return has been delivered showing the profits or gains or chargeable gains derived from those transactions, acts or operations..."
So, there's an obligation when you acquire a chargeable asset to have records. There's an obligation, as long as you have assets in respect of which you would be chargeable to tax on their disposal, to retain records such as will allow you to prepare a correct return. And there's an obligation, having disposed of an asset, to continue to hold records for a further period of 6 years, or indefinitely if you haven't filed a return. You can't get around the obligation to have records and linking documents, by simply holding the assets for long enough, and then plucking figures from thin air.
In the event that someone doesn't have records and is having to guesstimate material figures in respect of a CGT liability, they or their tax agent should be completing an Expression of Doubt to protect them from exposure to penalties.