Take it cum grano salis, but when I studied for semester Ireland in the late 80's I heard
There have been many attempts to naturalize eucalyptus in Ireland since the 30's, or at least that's what I was told when I was studying there in the late 80's. Something to do with local farmers wanting a fast growing tree which could supply both foliage for fodder and high quality wood for sale abroad. In the end demand for eucalyptus wood never materialized and killed the whole venture, but not before at least a dozen species were planted.
Some may have become naturalized but I think most of what you find today are the remains of failed commercial ventures. I seem to recall the tree Irish farmers planted the most was cider gum, mostly because its foliage makes decent fodder, but I've never seen any as big as that in the pictures, so take that with a pinch of salt.