This is a really dangerous one! The problem lies in the meeting of the complex Irish Offshore rules and the foreign tax treaties. Specifically, in the case of US ETFs, these are treated by the Irish-US tax treaty as producing dividends which entitle you to a 15% tax credit in Ireland. In this case the Revenue seems to treat them as securities (i.e. 41%+health levy on dividends and 20% disposal rate) if you wish to claim your credit.
Irish / EU ETFs are all treated under the 20% income /23% disposal regime. By the way, at the present moment, contrary to the comments of many, the securities treatment is far more beneficial if you plan to invest over the longer-term. This is primarily due to the 8-year deemed disposal rule which does not apply to securities as was introduced by the Finance Act 2006.
Naturally the above is only my opinion and I would like to hear whether anyone has had any real clarity from the Irish authorities on this!