I think it always feels surprising when people who've been together decades and bumble along without major drama, suddenly split.
I thought some of their banter was for the cameras and part of their persona. There were also things that didn't feel nice - thinly veiled and all that.
I think sometimes what can feel endearing/just who someone is, can get tiresome. I imagine being tired of how things were was more Ruth's reason than Eammon's.
They've both also had a big loss in the last couple years - Ruth's sister who was young & Eammon's mother whom he was very close to. Ruth seems to support her mother and sister's adult dc and Eammon is drawn to NI where his brothers are, and more importantly, his adult children and now grandchildren - though he also has a much younger adult child with Ruth who is in England - another reason for her to not want to move. Though at that age and with the loss of his mother & his health, it's also very normal to be nostalgic and want to be with family especially grandchildren. I mean we see that on MN all the time, grandparents moving to be nearer grandchildren.
It makes sense to me between all those reasons and perhaps a bit his politics/new job, that they just started to struggle to connect and to disconnect and want to move on. Add in health stuff and well, I imagine it was more Ruth wanting out than Eammon. I imagine she felt she had much more life to live...