Barring any songs in Disney live-action TV movies, that is.
Anyway, that song, “A Mhaighdean Bhan Uasal” (O Noble Lady/Fair Maiden) actually is sung by Emma Thompson (in dead-on authentic old style!) and the little girl voice actress, Peigi Barker.
As you can see, it’s a lullaby that’s actually original to the film, but composed in a very old-fashioned style. Obviously in character, it is something her mother composed before Merida had any brothers. Since the Brave world goes by primogeniture instead of the election from male heirs (or because the king doesn’t have any male relatives left besides his young sons, which might well be the case with someone made king for heroic deeds), she was obviously anticipating Merida possibly having to become a ruling queen.
The Disney Wiki translator doesn’t really bring across that Queen Elinor is addressing her own child (in the bardic style) as “O Sun” and “O Moon.” She’s the only heir at that moment, the only light toward a future of glory and happiness. (And of course Sun and Moon are feminine, in Gaelic, and the association of light with a red-headed baby is pretty useful.) There’s also wordplay, because “bean uasal” (noblewoman or lady) and “ban maighdean” (white/fair maiden) are both common phrases. So the queen is mashing them together on purpose, which is clever.
I’m still puzzled about Merida’s name. Mor’du is obviously “Mor Dhu” (great black one, or Mor the Black). Merida could be a Scottish form of the saint’s name “Merita”, or a form of Mhairi or Mairead, or a Spanish name, or…? Makes me feel better to know that others are puzzled, especially people from cities named Merida. But to be fair, the sources for feminine Scottish Gaelic names in early medieval times are not very plentiful.
“Elinor” and “Hubert” are French/English-influenced names, so clearly the Queen is from a more urban, port town, part of Scotland. Hubert is the saint of hunters, which is probably why that name. Hamish is James (Sheumais, really). Harris is the name of part of the Island of Lewis, out in the Hebrides; it’s either an Anglicized Gaelicized version of hearath (district) or haerri (high place) in Old Norse. So the triplets recap the three major influences on Scotland’s history, which is amusing.
St. Emerita of Rome was a virgin martyr, killed in Valerian’s persecution. Her feast is September 22. There’s also St. Emerentiana, the foster sister of St. Agnes who was caught praying at her grave. Emer is of course a famous figure in Irish and Scottish legend, because she was Cu Chulainn’s wife, and had a temper nearly to match hers. (In Scottish legend, she’s a much nicer, sweeter figure than in Irish legend; but in Scotland’s fairy tales, he’s usually a fearsome giant as well as a hero. Much like Finn McCool.) So it’s plausible that St. Emerita or Emerentiana might have become a Scottish name, given some good reason.
Mairi isn’t really found as an early Scottish name, just as in Ireland. They loved Mary, but it was a sort of taboo name, like Jesus. Later, of course, it was a very popular name. Like the song about the Four Maries: “There was Mary Seaton and Mary Beaton/ And Mary Carmichael and me.”
It could conceivably be a form of “Mariota,” another French-influenced name.
But I lean toward it being meant as a sort of Latinized version of Mairead, because that’s “Margaret” in Gaelic, and a common royal name. Also, St. Margaret slew dragons by keeping her Bible handy, so she sorta fits the theme.