The Voyage of Saint Brendan The Navigator a work in progress

Saint Brendan of Clonfert or Bréanainn of Clonfert (c. 484 – c. 577) (Irish: Naomh Breandán; Latin: Brendanus; Icelandic: (heilagur) Brandanus) called "the Navigator", "the Voyager", or "the Bold" is one of the early Irish monastic saints. He is chiefly renowned for his legendary quest to the "Isle of the Blessed," also called Saint Brendan's Island. The Voyage of Saint Brendan could be called an immram (Irish navigational story). He was one of the Twelve Apostles of Ireland.^[1]
Saint Brendan's feast day is celebrated on 16 May by the Roman Catholics, Anglicans, and Orthodox Christians.
^{[2]https://en.wikipedia.org/wiki/Brendan}

An immram (/ˈɪmrəm/; plural immrama; Irish: iomramh, IPA: [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.
 https://en.wikipedia.org/wiki/Immram

Immram
pl. immrama
M.Ir.? "rowing out"
The immram is a literary genre in medieval Irish literature, specifically refering to supernatural sea voyages. However, a distinction must be drawn between the echtrae--a journey to the Otherworld--and an immram--a supernatural voyage. For while one is pagan in aspect, the other is Christian.
It would seem that the confusion is caused by the text Immram Brain, in which the hero visits the Otherworld of Manannan mac Lír. The likely explanation is that the title is a mistake, or a confusion with the similar, Christian story of Saint Brendan, who is thought to have made a voyage to the Blessed Isles. Oddly, while the Latin name for Brendan's voyage is Nauigatio sancti Brendani abbatis, the Irish name is Betha Bréannain "The Life of Brendan"--not the Voyage of Brendan. For more on the confusion between echtrae and immram in regards to the Voyage of Bran, see echtrae.
Otherwise, the immrama have heavily Christian overtones:

Immram curaig Maíle Dúin: In which Máel Dúin sails to various islands, seeking his father's murderer. After seeing forty supernatural islands, he heads the words of a priest and forgives the murderer instead of enacting the typical Irish revenge motif.

Immram curaig Ua Chorra: In which three pirates, baptised as heathens and worshiping the devil, convert to Christianity after a mystical vision of Saint Michael the Archangel. They then repent for their sins by engaging on a supernatural voyage, where they see the usual talking birds and magical islands, all of which is perfectly Christian to the text. And so, while some have incorrectly described an immram as a voyage to the otherworld, it is not. Instead, it borrows certain elements of the Otherworld to work as a challenge of the faith of the voyager--not as a pagan description of the home of the gods. There is even some speculation that the immrama may have certain "real" islands in them, such as the Faroe Islands, or the Sargasso Sea, of all places.

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Immram Snédgusa ocus Meic Riagla: In which the kinsmen of Colm Cille visit various fantastic islands and preach the gospel. 

echtrae
pl. echtrai
OIr:"outing" usually translated as "adventure"
In Irish myth, an echtrae is an excursion to the Otherworld.
There has long been some confusion between the two genres of echtrai and immrama, due mainly to the mistitled Immram Brain, that is, The Voyage of Bran. I say it is misnamed, for the term immram--"rowing out"--is used to refer to sea voyages, particularly those of the monastic period, such as the Voyage of St. Brendan, the Voyage of Maelduin, or the Voyage of O'Curra. These are tales, although containing fantastic elements, which are not concerned with the otherworld; they are firmly set in this world, and specifically a Christianized world. In each immram, the hero is a Christian, and though he is faced with supernatural elements, he never changes his religion.
However, in The Voyage of Bran, we actually have an echtrae, not an immram. The echtrae is specifically concerned with the adventure of a hero in the Otherworld. It is firmly pagan in nature--gods and goddesses often beckon the hero on the voyage, which leads him to one of the many islands of the otherworld: Mag Mell, Tír na nÓg, Tír inna mBan, etc. The god is often Manannán mac Lír, and the goddesses one of his daughters, such as Niamh, and they are usually bearing a flowering branch of apples. The point of the echtrae is either to lure a hero to the Otherworld for eternity as a husband for one of the goddesses, such as the case of Connla or Oisín; or so as to impart some form of wisdom or power unto the hero, such as the case of Cormac mac Airt. Only the chosen return from the echtrai, and those who return against the will of the gods usually perish, having found that they had been away from Ireland for hundreds of years.
So what of the case of Immram Brain? Why is it called immram instead of echtrae? It was likely that Bran's story became confused with that of St. Brendan, and the term immram was passed on to Bran's story, despite its unsuited nature.