Death? Twilight? Mysterious disappearance?
I am sure it is written somewhere. But I do not remember...
She disappeared shortly before the Grand Tribunal of 799. See HoH:MC p.76, center column, next to last paragraph.
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That's about Merinita. Not Diedne, who the OP asked about.
Diedne was at least alive until 806, when she and Pralix had a fight. (Both of them survived)
I don't think we have any canonical informtation in 5e about what happened to Diedne the Founder.
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Oops, sorry! 
We can assume she lived to see the induction of Ordo Miscellanea as a Hermetic House; IIRC we are told Diedne was one of her most vocal detractors during that particular debate.
Where is that stated? I can easily believe Diedne would be opposed to anything Pralix was involved with, but I can't find anywhere saying it.
Thrice Told Tales, p. 136:
"It is notable that after the war with Damhan-Allaidh, Diedne was one of the most vocal detractors of Pralix and insisted that she be Marched for her crimes. "
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I read that very page, yet missed that sentence. 
We also know that by the year 848, Tremere was the last Founder standing.
So whatever happened to Diedne, it happened before that.
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TCI p. 112: Diedne’s ruthless campaign against the Irish druids lasted just under fifteen years; she boasted to the tribunal in 816 that there was not a single druid left in Ireland.
I'm not finding more recent mentions of Diedne herself so far.
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And yet in the contested Isle there are still druids in Ireland.
I suppose her boasting wasn't entirely correct then. Either that or they imported more druids from somewhere else. Or those who call themselves druids in 1220 are not of the same traditions that Diedne fought against.
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Diedne herself was supposed to have been a Druid as well, which means her house would have been Druids, and in Ireland...
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of course not; they are sleeping Diedne who just pretend to be ordinary hedge druids
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I suspect the "Mission Accomplished" banner was a trifle optimistic.
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ArM 5e p.9: "and from the traditions of the druids, brought by Diedne, a woman cast out from that religion, he learned to create magical effects spontaneously."
That seems to indicate that she was a druid until they gave her the boot.
And they probably did that because of her willingness to engage with Bonisagus and his Order.
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Which is really strange since druids never were big on the idea of excommunication. To my knowledge historically there was no way for someone to be kicked out of being a druid.
Considering how little hard knowledge we have about druids, that's not exactly surprising.
Bear in mind that if she didn't go about shouting join or die to every druid she met, she might have been less disliked.
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To me, the quote above implies that she was cast out first and then joined Bonisagus.
On the other hand, my impression of the druids is rather pacifist, while Diedne spawned a militarist tradition. That just might be enough cause to cast her out.
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There is a lot of confusing information about Druids between what Romans wrote about them and archeological evidence, but one thing that is certain is that they were not pacifists, or at least not total pacifists. They led Gaelic troops into battle and definitely killed people though evidence is inconclusive over whether it was executing convicted criminals or ritual human sacrifice- it is clear that it was not done often.
That being said they were also clearly not a cohesive religious structure that would go around throwing people out. It would be like saying that a Russian General had been thrown out of being a General by the other Generals because of war crimes. It simply wasn't how things were structured.
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