That's something yet to be defined (and soon).
I had been making some numbers. According to the HOH figures there are supossed to be around 68 Merinita magi out there. Our Saga is centered in the isle of Man, so around there we have Hibernia, Stonehenge and Loch Leglean, and after doing some numbers with something TimOB posted a while ago (here) I estimate 13 Merinita in Hibernia, 7 in Loch Leglean and 6 in Stonehenge, so we have 26 Merinita around and of course not all of them will secretly be Diedne. So I guess I could estimate the % of Diedne of between 10% and 50% of these magi. Taking a 20%, would mean that there are 5-6 diedne magi scattered over these 3 Tribunals (plus maybe other handful of Diedne who are actually infiltrated into Ex-Miscellanea, hiding into Regios and such). Taking a higher percentage would probably end in another Schism War scnenario against Merinita House, so they can't be very active if only because they have very small ranks... They probably are coherent because of their shared hidden lineage, but not very powerful, so the lost Mistery Cult/House information seems the most logical, and all these old Diedne covenants abandoned to be found or in ruins would be the 'remain to be found' part.
So it seems to me that the story would go in two directions. Our Actual Diedne will probably try to regain some of that lost knowledge, while the Tytalus, well, tries to earn a name in Hermetic History Books one way (like let's say exposing the remaining Diedne to the Order wrath) or another (like messing around to increase Diedne's House power and play political plots to gather hermetic, faerie and magic allies in order to start Schism War II).