The Earl was probably the most influential Irish-born person in Ireland at the time, and he had gone to London to sort out a dispute with the representative of King Henry VIII, Sir Anthony St Leger. St Leger apparently swore that only one of them would come back to Ireland, and on 17 October the Earl and thirty-five of his companions fell ill after a banquet at Ely house in Holborn. Eighteen of them died, including the Earl himself, on 28 October.
However, Edwards argues pretty convincingly that the poisoning must have been accidental, from contaminated food. He points out that the dispute with St Leger had been resolved by the time of Ormond's death - in fact, it was largely a misunderstanding engineered by the Lord Chancellor, who was thrown into the Fleet Prison as punishment. Indeed, as he was dying, the Earl actually named St Leger one of his executors, which indicates that he himself didn't think St Leger was responsible. On top of that, it seems that poisoning was a pretty unusual murder method in the sixteenth century.
Thinking it through, I guess this is supported by one more piece of circumstantial evidence: that it took the Earl eleven days to die. That surely suggests a biological cause of poisoning, rather than a chemical one, and I guess that even now you would hardly attempt to poison someone by slipping them some rotten meat, so it makes it more likely that the problem was caused accidentally. (Having said that, if the Earl survived the initial trauma, it was probably his doctors rather than the poison that actually killed him; cf President Garfield, etc.)
We know the Earl was reasonably clear-minded in those dying days because he made a number of deathbed provisions, including the appointment of St Leger as one of his deputies, setting funds aside for his surviving servants to return to Ireland, suitably dressed in black livery, and also making provision for the continuing education of the son of his steward, James White, who was apparently one of the first to die. The son, Nicholas White, later achieved high office in Elizabethan Ireland, and was also a direct ancestor of mine, which is why I developed an interst in the story.
That's enough for here; anyone who wants to see the full article let me know. And thanks again to pgmcc and his daughter for tracking it down for me.