1798: Aerial Navy seen in Croagh Patrick
On a quiet evening in 1798, James Hardiman (and 'hundreds' of other people, he tells us) watched as a fleet of ships apparently rose from the ground near the mountain of Croagh Patrick, Mayo, Ireland, into the sky.
Forty-eight years later in 1846, Hardiman wrote a translation of a strange event reported in 1161 which Hardiman considered to be explainable as an optical illusion; and this in turn reminded him of his 1798 experience, so he made note of it on the same page. Hardiman stated that many who witnessed the 1798 event thought it was supernatural... but Hardiman himself had come to believe it was just some sort of optical reflection in the sky of the fleet of Admiral Warren, who was in pursuit of a French squadron at that time off the west coast of Ireland.
The idea that a reflection of events far away could appear in the sky above an inland mountain is an interesting one that I don't know enough to confirm or refute at the moment. I will explore this possible explanation, but I will also try to find further sources for this incident, if in fact 'hundreds' of people witnessed it. For these reasons, this incident is marked as "Investigate" and is currently considered a Curiosity rather than an Anomaly.