Building society Nationwide UK Ireland has apologised to customers after they were incorrectly asked to provide personal information.
The financial firm said no data breach had occurred and this had been confirmed by the Data Protection Commissioner.
Nationwide UK Ireland's head office in Britain wrote to customers in Ireland requesting information about their tax residency.
They were asked whether they were tax resident in the UK or elsewhere and were asked to send certified copies of supporting documentation such as a passport, certificate of residency or national ID card.
A company statement said: "Some Nationwide UK (Ireland) customers recently received a letter in error from our head office in the United Kingdom regarding new international tax compliance regulations.
"We are contacting impacted customers to apologise for any inconvenience caused."
It said there was information on its website, and customers should make contact for any further information.
Nationwide UK Ireland is a subsidiary of Nationwide Building Society and the terms and conditions state that information may be shared within the society. Therefore, no data protection breach has occurred as a result.
Any information provided by Nationwide UK Ireland customers as a result of the mailing will be destroyed.