The airship of Clonmacnoise is the subject of a historical anecdote related in numerous medieval sources. Though the original report, in the Irish annals, simply mentioned an apparition of ships with their crews in the sky over Ireland in the 740s, later accounts through the Middle Ages progressively expanded on this with picturesque details. First the ships were reduced to one ship over Teltown from which a crewman threw and then recovered a fishing-spear. Then the scene shifted to the abbey of Clonmacnoise, and later to Britain, and the fishing-spear was changed to an anchor which snagged on some feature of a church. The sailor who climbed down to release it was also said to be in danger of drowning in the thicker air of this lower world. The story was retold by Seamus Heaney in a well-known poem collected in his 1991 volume, Seeing Things.

Several sets of Irish annals, those of Ulster, Tigernach, Clonmacnoise, and the Four Masters, all briefly mention a strange apparition; the Ulster Annals, for example, simply say that "ships with their crews were seen in the air". Though the annals differ as to the precise date, whether it was in 743, 744 or 748/9, they are nevertheless considered to be an early, if possibly second- or third-hand, description of the same event, an occurrence considered remarkable enough to be recorded. Naturally, the precise nature of these supposed ships cannot now be proved, though they have been variously interpreted as an unusual cloud-formation (such as the ship-shaped cloud recorded to have been seen by the 13th-century monks of St. Albans), a display of aurora borealis, or, by many ufologists, as evidence of an alien visitation. Most recently, it has been explained as an ocean mirage, a phenomenon which can make ships at sea appear to be above the horizon.

The story is repeated in the Book of Leinster as part of an account of events at the Teltown assembly: "Another wonder of the same assembly: seeing three ships voyaging in the air above them, when the men of Ireland were celebrating the assembly with Domnall son of Murchad", who reigned from 743 to 763. New details are given by an account in the Book of Ballymote. The ship (rather than ships) still appears at the Teltown fair assembly, but this time in the presence of the 10th-century king Congalach mac Maelmithig, and we are told that one of the crew threw a dart at a salmon, which fell among those gathered there. A man came down to retrieve the dart, but when one of those on the ground held on to it the stranger cried, "I am being drowned". Congalach ordered that the man be let alone, and he returned to the ship, swimming.

Patrick, a late-11th century bishop of Dublin, gives a Latin verse account of the story which closely parallels that in the Book of Ballymote, though leaving out the intervention of Congalach and the man on the ground.

A further development becomes evident in a version of the story preserved in Edinburgh, National Library of Scotland, Adv. MS 72.1.26. The manuscript is 15th or 16th century, but the text is not so easily dateable: Kenneth Hurlstone Jackson considered it perhaps 14th or 15th century, while John Carey assigned it to the later Middle Irish period, which ended c. 1200. The locale is moved from Teltown to the church at Clonmacnoise, and instead of a fishing-spear it is an anchor that is dropped. The priests seize it, but a man comes swimming down, and when the priests hold on to the anchor he protests that he is drowning. Then he swims back to the ship with his anchor.

The anchor form of the story spread outside Ireland, and can be found both in the chronicle of the late-12th century French abbot Geoffroy du Breuil, where the anchor is supposedly dropped onto London in 1122, and in Gervase of Tilbury's Otia Imperialia (completed c. 1211). Gervase tells us that, when leaving their local church somewhere in Britain one dark and cloudy day, parishioners saw a ship's anchor embedded in a heap of stones in the churchyard and a rope leading down to it from above. He continues:

The people were amazed, and while they discussed it among themselves, they saw the rope move as if [the crew] were struggling to free the anchor. When it would not budge for all their tugging, a voice was heard in the thick air, like the clamour of sailors vying to recover the thrown anchor. Nor was it long until, hope in the effectiveness of exertion having been exhausted, the sailors sent down one of themselves – who, as we have heard, dangling from the anchor rope, came down it hand over hand. When he was about to disengage the anchor, he was seized by bystanders: he gasped in the hands of his captors like a man lost in a shipwreck, and died suffocated in the moisture of our thicker air. But the sailors overhead, surmising that their comrade had drowned, cut the anchor rope after having waited for an hour, and sailed away leaving the anchor.

He ends by telling us that fittings for the church door, made from the anchor, can still be seen there.