The best discussion of other texts relevant to the Navigatio is the introduction and commentary of Orlandi and Guglielmetti (2014). Here I will mention only the most significant.
By far the most immediate literary influence on the Navigatio was the Latin Bible, as translated by St. Jerome (c. 345–420 CE), known as the Vulgate (editio vulgata) or the Gallicanum. From the New Testament, the most important books were the four Gospels (Matthew, Mark, Luke, John), and the Book of Revelation (The Apocalypse of John). From the Old Testament (The Hebrew Bible), by far the most important book was Psalms, recitation from which was the central activity of the daily office discussed above.
The Psalms are songs (Greek ψαλμοί) originally in Hebrew, traditionally ascribed to David. There are two separate conventions for numbering them. Jerome’s Latin translation (the Vulgate) followed the numbering of the Septuagint, the ancient Greek translation of the Hebrew Bible, which combined various psalms, while many modern Bibles usually follow the numbering of the original Hebrew text. When two numbers are given for the same Psalm, the first (by convention) is that of Jerome and the Septuagint; the second is that of the Hebrew Bible and many modern Bibles. Thus, the well-known Psalm 23 (“The Lord is my shepherd”) would appear as Psalm 22 (23). In the notes, I quote the English of the Douay-Rheims translation, as it is the closest standard translation to the Latin of the Vulgate; most Bibles in English are translations of the original Hebrew and Greek texts.
A number of other texts seem to have influenced particular passages in the Navigatio, either directly or indirectly. Perhaps the most important of these is Jerome’s short Life of St. Paul of Thebes (in Egypt), also known as St. Paul of the Desert or St. Paul the Hermit (died c. 360 CE, and not to be confused with the St. Paul, the Evangelist, who died c. 65 CE). In chapter 26, Brendan meets a hermit, also named Paul, whose life story recalls aspects of the life of Paul of Thebes. Other texts relating to the origins and practice of monasticism are important more generally, including the anonymous “History of Monks” translated into Latin by Rufinus (345–410 CE) and the Life of St. Antony of Egypt (d. 356 CE) by St. Athanasius, translated into Latin by Evagrius of Antioch.
Another important source for the Navigatio is the “Apocalypse of Paul,” also known in Latin as “The Vision of St. Paul” (Visio Pauli). This is the most influential example of what scholars now call an “apocryphal apocalypse.” Such texts describe “revelations” (“Apocalypse” is Greek for “revelation.”) like those in the New Testament Book of Revelation, also called the Apocalypse of John. They are also apocryphal (from the Greek for “hidden away”; they are also sometimes called pseudepigrapha) in the technical sense that they were not included in what are now the canonical Hebrew and Christian Bibles. They are not to be confused with “The Apocrypha” which are included in most editions of the Christian Bible (also called Deutero-canonical texts). The Visio Pauli was central to developing Christian conceptions of heaven and hell. Originally written in Greek in the fourth century and known in the west in Latin translations, it tells of a vision by St. Paul (the Evangelist, died c. 65 CE), in which he was taken on a tour of the rewards for the righteous in heaven and the sufferings of sinners in hell. The Visio Pauli is also the likely source for the striking concept that sinners are given Sundays off from the torments of Hell (Navigatio 25.9).
Far more obscure than the Visio Pauli is another apocryphal text surviving only at second hand, the “Book of Enoch and Elias.” This book—not to be confused with the also apocryphal “Books of Enoch” (1, 2, and 3 Enoch)—is summarized in Latin verse by yet another obscure figure, Godfrey of Viterbo (died c. 1191 CE), in a history of the world called the Pantheon or Liber Universalis. The complete Pantheon has never been printed, but an article by Mario Esposito (Esposito 2000 = 1960) quotes the sections summarizing the “Book of Enoch and Elias,” a manuscript of which Godfrey apparently found in the Abbey of Saint Matthew, on the western edge of (Celtic) Brittany. The “Book of Enoch and Elias” seems to have related how monks from Galilee, followers of St. Matthew, converted Spain and Brittany to Christianity and sailed west across the Atlantic in search of an earthly or spiritual paradise.
Texts in Old Irish show a particularly interest in ocean wandering, notably the echtrae (plural echtrai) and the imram (or immram, plural imrama or immrama). An echtrae (“Outing”) tells of a voyage or visit to a non-Christian otherworld; it is set in a land of fairies and magic, and the voyage itself is of only incidental importance. With an imram (“rowing-about”) the voyage itself is central, possessing a clear connection to Christian monasticism. Scholars sometimes treat the Navigatio as one of the surviving imrama, but it seems simpler to reserve that term for texts in Irish and regard the Navigatio as a Latin text that, uniquely, has much in common with the imrama. The dates of the extant echtrai and imrama are uncertain; in our surviving versions they may be later than the Navigatio, but one of the best known, the “Voyage of Maelduin” is often regarded as a source for the Navigatio. The two texts certainly have important incidents in common.
Finally, mention should be made of the Vita Sancti Brendani, known from five Latin and two Old Irish versions, none of which are earlier than the 12th century, and none of which are complete. Scholars date the lost original to the ninth or 10th century, and most think it was later than the Navigatio. The Vita contained many of the elements that would be expected in an ancient saint’s life, such as their education and remarkable deeds performed as a child. However, it also incorporated material found in the Navigatio; one version essentially quotes the Navigatio at length. The Vita also embellished the stories of the Navigatio, with (for example) an island with giant mice and a dead blonde maiden one hundred feet tall.