“…A cursory reading of some of popular medieval accounts of otherworld journeys […] reveals that Owen’s experiences in Saint Patrick’s Purgatory conformed to the conventions of well-worn literary genre…”

However, before we can talk about the literary genre, it would be useful to discuss some recurring “themes”, common narrative patterns, characterising primary models which structure the Irish “Eachtra”, that is, the stories of adventure such as the Eachtra Connla, the “Imram”, or better, those voluntary travels, such as the one taken by Maeldúin, Ua Chorra – prototype of the Christian Navigatio sancti Brandani – , by Snedgus and by Mac Riagla and the “fís”, real and irrefutable visions. Among these we must include those of the monks Fursa, Laisrén and Adamnán and those of the laymen such as Tundalo and the knight Owain to the Purgatory of Saint Patrick.

And if the reason for the descent into this hell is very old and the representation of an “Other world” has been researched in and of itself as well as in relationship to the Divine Comedy “in the Celtic tradition a visit to the Other world, the “Tir Tairgire” or “Tír na nÓg” was an integral part of the Eachtra or adventure story” and precisely within this tradition, having quickly become a Christian one, the legend of the Purgatory of Saint Patrick is born, codified into three different Latin versions “represents in some way the birth of literature about the Purgatory”, even though the identification of a third intermediate reign was already present at the time of the Greeks. Furthermore, Blochet had noted how the Purgatory of Saint Patrick offered certain points of commonality with the oriental legends of the Ascension: in particular with the Arda Viraf and the oriental vision of Hell. Among the motifs in the oriental ascendence was that of the bridge ( the Persian cinvat already present in the Visio Pauli), a metaphoric theme that in other texts might be similar to the hebrew-semitic idea of the ladder (also present in the vision of Adamnán) assuming a double nature of the sometimes infernal element (such as is the case of the Visio Alberici: “Post hec vidi scalam ferream (…). Pedes autem per scalam ascendentium et descendentium exurebantur in illis gradibus ignitis”, sometimes purgatorial one (in the Visio Pauli,and later in the Purgatory of Saint Patrick).

All of this describes only in small part the importance that this “zipper legend” had and its popularisation on the European continent.

It tells the story of the monk Patrick, captured by pirates and transported to Ireland, later becoming an apostle for those peoples “cum tota Britannia algore incredulitas rigesceret”. The Chronica Hybernienses recounts how one day, Christ himself had led the monk, who in the Vita Tertia has the reputation of being a thaumaturgical hero, a miracle worker, to a legendary cave, a sort of well which tradition identifies as being located in Lake Derg or the Red Lake in the county of Donegal, which, by passing through it, one can enter into the hereafter: “De hoc quoque purgatorio et eius origine quod sequitur tradunt veteres historiae hybernienses”. The story of the Irish knight, Owain, who ventures into that cavern at one time indicated to bishop Patrick is told by Matteo Paris in his column published in his Chronica maiora in the year 1153 and later in the Tractatus de Purgatorio sancti Patricii by Henricus Saltereiensis written between the years 1170 and 1185. The legend, started in Ireland, had various versions in different languages, middle English, Anglo-Norman, Provençal, old French and Spanish. They all tell the story of the foundation of Purgatory and the visit made by knight Owain there, how he entered into the cavern, after the preliminary rituals and repentance, how he had been warned by men dressed in white about the tortures and torments that he would encounter, what punishment he would have suffered before arriving in the Heaven on earth from which he was to be excluded. Those who were allowed to enter into the mysterious cavern located on an island in the Lake Derg were to report back to the others, upon their return, exactly what they saw and heard. The narration finished with the acts of the archives of the abbey, but not one of these summations ever arrived all the way to our part of the world. There are many versions of the legend and they are told in many languages.

http://web.classicistranieri.com/it-danteide/publications/patrick.html