The ancient writers, in doubt as to the earliest origin of the Gauls, have left an incomplete account of the matter, but later Timagenes, a true Greek in accuracy as well as language, collected out of various books these facts that had been long forgotten; which, following his authority, and avoiding any obscurity, I shall state clearly and plainly. Some asserted that the people first seen in these regions were Aborigines, called Celts from the name of a beloved king, and Galatae (for so the Greek language terms the Gauls) from the name of his mother. Others state that the Dorians, following the earlier Hercules, settled in the lands bordering on the Ocean. The Drysidae (i.e. druids) say that a part of the people was in fact indigenous, but that others also poured in from the remote islands and the regions across the Rhine, driven from their homes by continual wars and by the inundation of the stormy sea.
- Ammianus Marcellinus, Roman History [15.9.2-4].
This vision of a portion of the great nation of the Gauls coming in from far away islands is certainly alluring, particularly with reference to Atlantis (and the prominence of western islands in the worldview of the later Celtic peoples of Ireland and elsewhere). It is equally certain, however, that this statement, which Ammian takes on the authority of Timagenes, has nothing to do with Plato's Atlantis: no less a personage than the great Roman general and statesman Julius Caesar [Commentarii de Bello Gallico 6.13] is good enough to supply the name of the island in which the concepts of druidry were said to have developed: -
[T]he druids [...] are engaged in things sacred, conduct the public and the private sacrifices, and interpret all matters of religion. To these a large number of the young men resort for the purpose of instruction, and they [the Druids] are in great honor among them. For they determine respecting almost all controversies, public and private; and if any crime has been perpetrated, if murder has been committed, if there be any dispute about an inheritance, if any about boundaries, these same persons decide it; they decree rewards and punishments; if any one, either in a private or public capacity, has not submitted to their decision, they interdict him from the sacrifices. This among them is the most heavy punishment. [...] Over all these Druids one presides, who possesses supreme authority among them. Upon his death, if any individual among the rest is pre-eminent in dignity, he succeeds; but, if there are many equal, the election is made by the suffrages of the Druids; sometimes they even contend for the presidency with arms. These assemble at a fixed period of the year in a consecrated place in the territories of the Carnutes, which is reckoned the central region of the whole of Gaul. Hither all, who have disputes, assemble from every part, and submit to their decrees and determinations. This institution is supposed to have been devised in Britain, and to have been brought over from it into Gaul.
Though Britain's inhabitants are no strangers to water, the island is still above sea level. The account also finds echoes in the Irish myth of the Tuatha Dé Danann and their sojourn in the so-called "northern islands of Greece" (where "Greece is likely a gloss dating from a time after the initial development of the tradition). From four cities - named as Falias, Gorias, Finias and Murias - they were said to have brought four priceless magical artefacts into Ireland, whence they appeared veiled in mist, generations after their ancestors had departed. Of course, the Tuatha Dé appear to be euhemerised Celtic deities worshipped by the pre-Christian Irish, though Celtic mythology is - as stated above - certainly cognizant of mystical islands.
In addition to the discourse above, Ammian provides a classification of earthquakes [17.7.13]: -
Now earthquakes take place in four manners: either they are brasmatiae which raise up the ground in a terrible manner, and throw vast masses up to the surface, as in Asia, Delos arose, and Hiera; and also Anaphe and Rhodes, which has at different times been called Ophiusa and Pelagia, and was once watered with a shower of gold; and Eleusis in Boeotia, and the Hellenian islands in the Tyrrhenian sea, and many other islands. Or they are climatiae, which, with a slanting and oblique blow, level cities, edifices, and mountains. Or chasmatiae which suddenly, by a violent motion, open huge mouths, and so swallow up portions of the earth, as in the Atlantic sea, on the coast of Europe, a large island was swallowed up, and in the Crissaean Gulf, Helice and Bura, and in Italy, in the Ciminian district, the town of Saccumum was swallowed up in a deep gulf and hidden in everlasting darkness. And among these three kinds of earthquakes, myaemotiae are heard with a threatening roar, when the elements either spring apart, their joints being broken, or again resettle in their former places, when the earth also settles back; for then it cannot be but that crashes and roars of the earth should resound with bull-like bellowings. Let us now return to our original subject.
Whether or not Ammian has Atlantis in mind here is moot: he postdates Plato, so obviously he - like so many other supposed purveyors of information about Atlantis - would have heard of such a destruction through Plato.