OK, so a bit of background. I work part time as a tour guide in Yorkshire, and my usual stamping ground includes Whitby. One important event I mention is the Synod of Whitby, at which the contest between the Celtic/Irish traditions maintained by the monks at Iona and the Latin traditions established in the Kingdom of Northumbria was resolved in favour of the latter.
So once we get there and everyone's heading off to have their fish & chips and goth ice-creams, a chap pulls me aside and says, "by the way, did you know that the Irish tradition wasn't a catholic one, it was Eastern Orthodox?" He was himself an Orthodox priest, from Texas, though in "plain clothes" on holiday. "We don't think of Saint Wilfred as quite such a saint, of course! The Irish were first converted by Orthodox Christians fleeing North Africa, so that was the tradition they had. It's why they calculated the date of Easter differently."
So yeah. I made polite noises and said I'd look into it more because I hadn't heard about it - and the couple both seemed very well-informed and learned about things generally. I gather that the distinction between "orthodox" and "catholic" at this point in history (7th Century AD) is not really a defined schism yet anyway, but I'm coming up blank on any attempt to look into what he said...
It's an anachronism, you're correct.
Recently a lot of interest in Orthodox v. Catholic dogma has come up on the internet; amusingly, a lot of it seems to stem from video games like Crusader Kings. (Which says something about where our educational priorities lie, doesn't it?)
Which is what led me to a very similar question myself, with basically the same answer you already suspect: you can't really draw a meangingful, theological use of the label "Eastern Orthodox," as distinct from the Roman Catholic, until at the earliest the Second Council of Nicaea, in 787. This is the last of the Ecumenical Councils, producing dogma accepted by both the Greek and Latin churches. After this, their councils and conclaves are separate. Prior to that date, while they have differences in language and geography, they are fundamentally different arms of a single church. After that, they begin diverging farther and farther in theology. And since the Synod of Whitby was almost a century prior to this date, I'd say that it's a meaningless exercise to 'claim' the early Irish as belonging to either Catholic or Greek traditions, sort of akin to modern people projecting political parties onto Jesus or stuff of the sort. It's not wrong, per se. I can see why a devout man would say it. It's just not really meaningful in any way, as our idea of "Orthodox Christians" isn't reflective of early Irish Christianity.
