The best Aengus story is a legal argument. He wants Brú na Bóinne, the great mound on the Boyne, and he asks its occupant for the loan of it for a day and a night. That is granted. When he is asked to give it back he refuses, on the grounds that all of time consists of days and nights, and what he was given was therefore the lot.
Nobody in the text disputes the reasoning. This is a literature produced by people trained in law, and the joke lands because the pleading is technically sound.
A pregnancy that lasted one afternoon
His mother is Boann, whose husband is Elcmar; his father is the Dagda. To conceal the affair, the Dagda sends Elcmar away on an errand and holds the sun still, so that nine months pass in what the husband experiences as a single day. The child is conceived at dawn and born before evening.
That is the explanation Tochmarc Étaíne gives for the name Mac Óc, the young son: young, because he was conceived and born between morning and night. The pun is doing real work — it makes his youth a permanent condition rather than a stage, which is why he is the one figure in the Irish material who never seems to get any older.
He is fostered by Midir, and grows up not knowing his parentage; the discovery, in the standard way of these tales, arrives as an insult from another boy.
The dream, the swans, and the lake
Aislinge Óenguso, the Dream of Óengus, is short, strange and unlike anything else in the corpus. A woman appears at his bedside. She is gone before he can speak, and he falls ill with love for someone he cannot name and has no evidence exists. His family conduct what amounts to a nationwide missing-person search, and eventually she is identified: Caer Ibormeith, daughter of a lord of Connacht, who lives at Loch Bél Dracon.
The complication is that she spends alternate years as a swan, changing at Samhain, and her father cannot compel her. Aengus goes to the lake, finds a hundred and fifty birds in pairs joined by silver chains, calls to her, and is told he may have her if he becomes a swan too. He does. They circle the lake three times and the singing sends everyone within hearing to sleep for three days and nights.
The tale has no violence, no abduction and no test of strength. It resolves entirely through identification and consent, and it is one of the earliest European narratives to make a dream-image the whole engine of a plot.
What he does for other people
He is a rescuer more often than a protagonist. When Étaín is turned into a fly, Aengus builds her a sunlit bower of glass and carries her about in it, which is the only kindness in that grim wooing-tale. When Diarmuid, his foster-son, is left to die by Fionn, Aengus takes the body to the Brú and puts a soul into it so that he can speak with him each day — a piece of grief-management with no parallel elsewhere in Irish literature.
Both episodes place him on the side of the person the story is destroying. It is a consistent characterisation across texts that otherwise have nothing to do with each other.
The mound, and a caution about dates
Brú na Bóinne is a real place and a spectacular one: a Neolithic passage tomb complex whose main mound was constructed around 3200 BCE, with a roof-box above the entrance that admits sunlight to the chamber at midwinter sunrise. That opening was identified and its function confirmed by Michael O'Kelly's excavation in the 1960s.
The people who built it lived roughly four thousand years before the tale of the day-and-night bargain was written down, and did not speak a Celtic language. Aengus's tenancy is a medieval Irish claim about a monument that was already impossibly ancient when the claim was made. Popular accounts that make him a god of the winter solstice are combining a Neolithic engineering fact with a medieval story that never mentions it.
Yeats, and the birds that may not exist
Two things attached to him in the modern period are worth separating from the medieval record.
The first is W. B. Yeats's "The Song of Wandering Aengus" of 1899, with its hazel wand, its trout that becomes a glimmering girl, and its closing silver and golden apples. The poem is superb and it is not the Irish story; Yeats borrowed the name and the atmosphere for a plot of his own devising. It has nevertheless done more than any manuscript to fix him in the English-speaking imagination as a figure of longing.
The second is the image, repeated in almost every popular description of him, of four birds circling his head — his kisses made visible. It is charming, it is everywhere, and it is very hard to locate in a medieval text. It appears to enter through Victorian and Edwardian retellings and to have been copied outward from there. Until someone produces the manuscript reference, it should be described as a modern accretion rather than as something the Irish sources say.
His name, by contrast, is thoroughly real and thoroughly historical. Óengus, from an earlier compound meaning something like "one choice" or "single vigour", was borne by Pictish kings, by Munster kings and by clerics, and survives as Aonghus, Angus and Innes — a Scottish county carries it. Readers curious about the name in current use will find name histories covers that lineage; the mythological branch of the family is drawn on the Celtic family tree.
Is he a god of love in any sense the sources support?
Only obliquely. The texts give him one love story of his own and several in which he intervenes for others, which is not the same as a domain. Nothing calls him a god of love, no ritual is recorded, and the label appears to be back-formed from the dream-tale by writers looking for an Irish Eros. What the material actually credits him with is persistence in the face of an impossible object, which is a narrower and more interesting thing.
Why is he called Óengus of the Brú when the mound belongs to a family?
Because the trick is the point of his connection to it. In some versions the man he outwits is Elcmar, his mother's husband, and in others the Dagda himself, which changes the story from a stranger's dispossession into a son taking his father's house by a technicality. The manuscripts preserve both, and the inconsistency is the ordinary condition of Irish genealogical narrative rather than a puzzle to be solved.
Does the swan transformation connect to the Children of Lir?
They are separate tales with a shared image, and the shared image is not accidental — swans are the standard Irish sign that a person is not currently in their own shape, and the silver chain linking a transformed pair recurs in more than one text. But the mechanisms differ completely: Caer's change is cyclical, inherent and voluntary within its cycle, while the swan-children are cursed by a stepmother and cannot end it. Reading one as a variant of the other flattens both.