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