An immram (/ˈɪmrəm/; plural immrama; Irish: iomramh, pronounced [ˈʊmˠɾˠəw], voyage) is a class of Old Irish tales concerning a hero’s sea journey to the Otherworld (see Tír na nÓg and Mag Mell). Written in the Christian era and essentially Christian in aspect, they preserve elements of Irish mythology.
The immrama are identifiable by their focus on the exploits of the heroes during their search for the Otherworld, located in these cases in the islands far to the west of Ireland. The hero sets out on his voyage for the sake of adventure or to fulfill his destiny, and generally stops on other fantastic islands before reaching his destination. He may or may not be able to return home again.
Source: Immram – Wikipedia
Tim B.
“The immrama are generally confused with a similar Irish genre, the echtrae or “adventure”. Both types of story involve a hero’s journey to an “otherworld”, whether a Christian paradise, a fairyland, the land of the gods or a utopia. They are distinguished by date; echtrai are older, dating from the 7th century, while the earliest immram dates only to the 8th century.[1] David Dumville argues that the immrama include more Christian thinking than the more pagan genre of echtrae,[2] and that, whereas the purpose of the echtrai is to enhance understanding of the old gods and the land in which they live, in an immram these pagan elements occur as a challenge to the hero’s faith. In an echtrae the protagonist only ever goes to one location and may arrive in the otherworld with no explanation of the journey, whereas in an immram the hero always has multiple adventures on several islands.[2]”
Tim B.
“Perhaps the largest piece of evidence that immram are Christian works is that the characters in the story are generally wandering priests, monks, and nuns, or at least related to them. “
Tim B.
https://en.wikipedia.org/wiki/Echtra