La respuesta de Interactive Brokers es que han hecho una consulta a las autoridades Irlandesas para ver si enviando el documento 8-3-6 firmado por el titular de la cuenta es suficiente.
As it currently stands, IBIE are required by the Irish Tax Revenue to have the Form 8-3-6 completed and stamped by your local Tax Authority.
We have reached out to the Irish Revenue authority to see if such forms as the Proof of Residency forms from the Spanish Tax Authorities are acceptable in lieu of signing section 2 of the form and are awaiting a decision.
With this in mind can you please complete and submit the Form 8-3-6 so we will at least have this on file in tandem with your residency form.
Once we receive any update from the Irish Revenue we can then advise if anything further is required from you.