If don't have the best of relationships with the person involved, what do you have to lose by going to them and telling them that their comment about making you an Executor of their estate is giving you cause for concern, and that you do not want to do it. If is ticks them off, so what? It was darn rude of them to make you the Executor without getting your ok in the first place. You mentioned that you told them that you do not want to be a benificiary of the will. That is not the same thing as being the executor. As you did not mention that at the time, perhaps the person in question is not aware of your reluctance to be neither a benificiary nor an executor of his estate? Perhaps they took your silence on the matter as an ok.
Point out to them that if they go ahead with their plans, you will decline to be the Executor when the time comes. An unnecessary amount of money will then go to Solicitors, as they and the beneficiaries try to figure out something else to do. Do they really want to waste that time and money? I doubt if they will.
If it were me, I would not take the traditionally Irish way out, by sticking mind head in the sand, and presuming that it will all go away. It won't. Yes, you can walk away from the duties of being an Executor. You can not walk away from the hard feelings it may create within the family after wards. Who knows, the person involved may even appreciate your honesty, especially if you bring it up in a tactful way and let him/her know that your reluctance to be their Executor is not a reflection on your feelings for them. You just don't think that you are the right man/woman for the job due to lack of qualifications/experience/other commitments etc etc