Same letter received here, but I don't think it is in error. The covering letter states "As our records show you have an indicator of tax residency outside of the UK, we need you to follow the steps below". The requirement is to fill in the enclosed form and supply supporting docs.
When you look at the form, it functions either for a UK or non - UK resident -- Qs 1 & 2 determine whether you are UK, US or "other" (including Ireland) tax resident. Once your residency is determined, if it is non - UK, you are asked to identify your country/countries of residence, give your TINs (presumably PPS for Ireland) and supply supporting documentation.
The PITA is that they ask for "certified" copies, which usually comes at a cost (solicitor, notary, bank official etc).
The sanction, if you don't reply, seems to be to inform HMRC (UK tax authorities), which probably has no negative consequences for an Irish resident. My nagging fear is that if you continue not to respond, they may just close the account. I will be responding, but my first shot will be to send simple copies without having them certified.
The Royal Mail prepaid envelope suggests that they might be mailshotting all clients, UK or otherwise, to collect the info and can't differentiate between the UK/non UK clients at envelope stuffing time.
Edit : the prepaid envelope is international business reply and will be accepted within the An Post system for UK delivery.