The Irish otherworld has too many names for a single place, and the one everybody knows is among the youngest of them.
Early and medieval Irish texts prefer Mag Mell, the plain of delights; Tír Tairngire, the land of promise; Tír na mBeo, the land of the living; Emain Ablach, the isle of apple trees. Tír na nÓg, the land of the young, is rare in the older material and owes its dominance to an eighteenth-century poem and to the school curriculum that made that poem famous. Using it as the standard term is convenient and slightly anachronistic, and it is worth knowing which is which.
What the voyage tales describe
The genre matters more than the name. Irish literature has two related forms: the echtrae, an adventure in which someone is summoned into the otherworld, and the immram, a rowing-about in which a crew sails west and encounters a series of islands. Between them they contain most of what is actually known about this material.
Immram Brain opens with a woman appearing in a closed hall holding a silver branch of apple blossom, singing about a distant island supported by four pillars of bronze, where there is no grief, no sickness, no death and no harsh sound, where a game goes on continuously under a tree, and where a wave of glass breaks on the shore. When she leaves, the branch goes with her. Bran takes twenty-seven men and sails.
The islands they find are not one paradise but a sequence of strange and specific places — an island of laughter whose inhabitants cannot stop, an island of women where the crew stay for what feels like a year. Immram Maele Dúin extends the pattern to thirty-odd islands, each with a single rule of its own, and reads like a catalogue of thought experiments.
The cost of coming back
This is the tradition's most consistent idea, and it works in both directions.
Bran's crew return and call to people on the shore, who do not know them but say that the voyage of Bran is among their ancient stories. One man leaps from the boat to the sand and immediately turns to ash, as though he had been dead for centuries. Bran does not land. He tells his story from the water, writes it in ogham, and sails away.
The same mechanism, told the other way round, ends Oisín's three hundred years in a single moment when his foot touches the ground. It is not a moral about the dangers of pleasure. It is a statement about incompatible timelines: the otherworld is not later or earlier, it is running at a different rate, and contact resolves the difference all at once.
Time also runs the other way in some texts, with a night in a mound consuming a year outside, and in Pwyll's Welsh equivalent a year passes with no distortion at all. There is no rule, which is itself informative — this is a literary tradition exploring an idea rather than a doctrine with fixed terms.
Not a heaven, and not for the dead
The single most common modern error is to describe Tír na nÓg as the Celtic afterlife.
The people who go there in the texts are alive: sailors, kings' sons, warriors summoned by a woman with a branch. They eat, they marry, they get bored, they come home. Nobody dies to get there and nobody is judged on arrival. Early Irish literature is in fact remarkably reticent about where the dead go, which is one reason the otherworld gets pressed into the role.
Two other terms should be kept out of it. The Summerland is a Theosophical and later Wiccan concept from the nineteenth and twentieth centuries, with no Irish ancestry. And the idea of Tír na nÓg as a reward for virtue is Christian framing imported wholesale.
Where the otherworld genuinely does connect to the dead is through the mounds. When the Tuatha Dé Danann withdraw into the síde, they take up residence inside prehistoric burial monuments — so the Irish otherworld has an address in real graves as well as beyond a real horizon, and the two locations are never reconciled.
Saint Brendan, and an island on the maps for five hundred years
The Christian branch of this literature is the Navigatio Sancti Brendani, composed around the ninth or tenth century, in which a monk and his crew sail west in search of the Land of Promise of the Saints and encounter an island that turns out to be a whale, a column of crystal in the sea, and an outcast Judas on a rock. It was one of the most widely copied texts of the Middle Ages, translated across Europe, and it is unmistakably the same genre wearing a habit.
It also had practical consequences. Saint Brendan's Isle appeared on European charts for centuries, and Hy-Brasil — a circular island west of Ireland, drawn on maps from the early fourteenth century and still marked on some into the nineteenth — was searched for repeatedly by real expeditions. A literary otherworld became a cartographic error that men sailed to find.
The phrase now
Tír na nÓg has become one of the most portable pieces of Irish culture: the title of poems, films, songs and an enormous number of pubs on several continents, and a standard shorthand in Irish public language for an idealised past. That usage is real and recent, and it sits on top of a body of medieval writing that was far stranger, far less consoling, and much more interested in what happens to a man who steps off a boat. The Celtic tradition hub sets out how those layers separate.
Who is the ferryman?
There isn't one, in the sense the question implies. Manannán meets Bran mid-ocean and explains where he really is, and elsewhere provides the means of crossing, but nobody in Irish literature pays a fare or is conducted to judgement. Travellers get there by their own boat, by invitation, or by following an animal into a mound — three ordinary methods, none of them ritual.
Why apples?
Because the apple branch is the standard sign that an otherworld summons is genuine, appearing in silver, in bloom, or bearing fruit and blossom simultaneously. Emain Ablach, the isle of apple trees, carries the same element, and it is the name usually connected with Arthurian Avalon by way of Geoffrey of Monmouth's Latin insula Avallonis. The connection between the Irish and the Arthurian island is by name and imagery rather than by any narrative link.
Is there an entrance anybody can point to?
Several are claimed, and the honest position is that the texts locate it inconsistently on purpose: west across the sea, under lakes, inside particular mounds, and at Oweynagat by Rathcroghan, which a medieval source calls a door. What the sources agree on is that the boundary is thin at Samhain and that it is crossed accidentally at least as often as deliberately — a geography of near-misses rather than of gates.