This is the reply I received from the IRS. In short: W-9 wrong, W-8BEN correct.
Thank you for your inquiry. We apologize for the delay in our response. Yes the United States and Ireland have entered into a tax treaty to avoid double taxation. You can find the treaty along with a technical explanation of the treaty on our web site. If you are a U.S. nonresident alien, the Form W-9, Request for Taxpayer Identification Number and Certification, would not be the appropriate form to you to file. Please refer to the following information. Form W-9, Request for Taxpayer Identification Number and Certification, is used to request the taxpayer identification number (TIN) of a U.S. person (including a resident alien) and to request certain certifications and claims for exemption. Withholding agents may require signed Forms W-9 from U.S. exempt recipients to overcome any presumptions of foreign status.
You must give Form W-8BEN, Certificate of Foreign Status of Beneficial Owner for United States Tax Withholding, to the withholding agent or payer (not the I.R.S.) if you are a foreign person and you are the beneficial owner of an amount subject to withholding. Submit Form W-8BEN when requested by the withholding agent or payer whether or not you are claiming a reduced rate of, or exemption from, withholding. A foreign person includes a nonresident alien individual, a foreign corporation, a foreign partnership, a foreign trust, a foreign estate, and any other person that is not a U.S. person. It also includes a foreign branch or office of a U.S. financial institution or U.S. clearing organization if the foreign branch is a qualified intermediary. Generally, a payment to a U.S. branch of a foreign person is a payment to a foreign person. Failure to provide a Form W-8BEN when requested may lead to withholding at a 30% rate (foreign-person withholding) or the backup-withholding rate. For more information, please refer to the instructions for the above mentioned forms. Thank you for using our system.