The Revenue Commissioners' Capital Acquisitions Tax Manual - Part 5 Discretionary Trust Tax (available on Revenue's web site) tells you all you need to know about discretionary trust tax. If you send the relevant section of your late grandfather's will to Revenue's Capital Acquisitions Tax Unit, Athy, Co. Kildare, they will tell you if a discretionary trust exists and what the tax obligations are. You could also ask your solicitor for an opinion but, in my experience, Revenue is the deciding authority on whether a discretionary trust exists or not.