I was wondering, the invasion of Tamriel by the Daedra in the third era was a universal event. How did this affect the Summerset Isles?

We've gotten canon on Daedra and Argonia and Daedra and Morrowind, but no news from the Summerset Isles. I assume the strong altmer magicks would have stopped the gates from opening in the first place, or else Dagon didn't bother with them because it would have been such a massive effort to conquer them without an existence-denying plane(t)-orbited robot.

Is their any information, canon or not, about this? Feel free to put forward your speculation.

I would assume Dagon's top priorities would be the White Gold Tower. After having secured Cyrodiil, he would have moved on to the rest.

The rest of the world probly saw much smaller scale invasions.

Yeah, Summerset Isles would have been a big project, probably bigger than a frontal attack on Cyrodil, since the Summerset Isles had it's [censored] together at the time. He must have been leaving it for later.

The only thing we hear is rumors of some Altmer turning to Daedra worship. Combined with Mankar Camoran's status at the head of the Mythic Dawn, I almost start to think that the MD was a patsy organization that allowed the Altmer to hurt their enemies, while gathering up and generating an excuse to execute cultural deviants. Obviously, if that was the case, there'd have been no serious attacks against Summerset herself.