This idea came to me inspired by the song Willy's Lady, which has two fairly good versions, one by Martin Carthy, and another one by Lady Maisery.
The story the song narrates consists of a man who crosses the water looking for a wife. He finds a woman with blond hair and brings her home, but his mother disagrees with the choice. As she's with child, she's unable to give birth. Willy goes three times to his mother, offering different exorbitant gifts, in exchange for her to lift her spell on her wife.
A blind character, probably a servant, gives them the idea of shaping an infant made of wax, and inviting Willy's mother to the christening. There, he keeps close to her, and hears her wonder who has carried out certain actions, necessary to undo the spell. That is how they become free from it, and they live happily ever after.
In the Kingdom of Dal Riada, in the monastic community of Iona (or some similar one), studies a noble lady, who, in accordance to Gaelic custom, may have exercise some form of actual rule (administrative or even military). Let us say she falls in love with one of the monks, perhaps from Eire, and is with child. Before they marry, a viking raid on the monastery results in looting, death, and the capture of several monks including her lover as a slave.
The clan's council is reluctant to admit the noble woman's position of rule, as she has an effectively fatherless child who might become a succession problem, as well as a potential drain on clan resources. In spite of which, because of her particular gifts (could be administrative, military, or perhaps on the realm of knowledge and the arcane arts, to go with the rest) she is permited to continue exercising power. She raises the child, which has no right to inherit her wealth, so he becomes a noted navigator and trader.
During his voyages, he grows to admire viking society, perhaps because of their military prowess, or simply because it is easier to reach high status from low birth through the pursuit of raiding and commerce than in a more regulated and lawful pastoral society as we might conceive of Dal Riada. He woos his wife (this could take place in any of the Norse settlements, as I believe Skaldic traditions existed in all of them, though Iceland was notable in preserving them) and with him brings back a large dowry consisting in a hoard of precious metal (perhaps hacksilver and some traded objects and coinage from Byzantium), and some animals of better quality than one could get in north western Britain, obtained from raids further south. Hence the origin of the cup, horse and gown (sheep?). ^(These are the gifts offered to his mother in the song.)
His mother opposes the marriage on 4 separate grounds which to her are perfectly rational: 1) she's a foreigner, not a clan member, and it will make it even harder to contrive to get him settled in clan lands; 2) she's Norse, from the people who enslaved her husband to be and her son's father, 3) her own husband to be was foreign, and it caused her problems she doesn't want for her child; and 4) with reservations as to the chronology, she may not be Christian, or at least not the right kind of Christian (celtic church).
However, his mother pretends to acquiesce to the marriage as he gives solution to some of the problems, and befriends her daughter-in-law, arranging her dresses and hair to help her adapt to the new environment. It is of course at this point when she crafts her spell, based on her knowledge of magic gained in her monastic studies. (From the song: 9 witch knots on her locks, combs of care, a kid who runs under her bed, and a tightened left shoe.)
There we get the scene with the maiden pregnant and incapable of giving birth. They ask assistance from everyone: wise women, the church, and the ollamhs, if they still exist. Afterwards they attempt to trade for it with her mother, offering all things of her dowry, to no avail.
A blind beggar who goes from fort to fort asking for hospitality and telling stories calls by, and the maiden, who is not aware of the traditions of the land, rejects him. In mediaeval Scotland it was common to keep a blind or disabled servant in the household for luck, as it was thought they could access alternate perceptions. He further seeks hospitality and is further rejected, and she commands her servants to beat him. On the coming day, as her husband is at the fort, the blind beggar comes again and recounts what treatment he has been receiving, and how he has heard the troubles which ail them. He also tells them he has the solution, but he will not give it without due compensation, which involves a monetary reward, a public apology, and a kiss. She's very reluctant to grant it, but the spell is almost to its end and she can't bear it anymore, so in the end she accepts, and the blind man tells them of the solution.
Afterwards, the Christening day, and the revelation of the nature of the spell, and hence its undoing. The mother, now considered a bringer of evil spells loses her influence, and is sent in a rudderless ship, to the traditional celtic judgement of God on kinslayers and witches.
Husband and wife, free from the spell and with their newborn safe, go back to the wife's father's Hall, to give account of the situation. The wife thinks that such a strange tale may actually increase his prestige, if it is duly told as a skald would, which she is qualified for. There at the hall, she tells the story and indeed people are impressed about their steadiness and resourcefulness in defeating the witch.
Having done good trading and received the praise of the hall's warriors for their song, they are soon to part. Before that, a thrall enters their quarters. It is his mother, enslaved by the Norse having recovered her rudderless ship. On finding her there, they manage to reconcile, and he offers to buy her and bring her back, but she refuses, saying her son should not acknowledge she could be an object of ownership. He offers then to forego all his gains there and steal her from the hall. Although his wife is horrified at the thought, the audacity involved gains her, and his mother has after all begged their forgiveness. However, she also refuses this, and says that God has so decreed, and she will stay a thrall there until the end of her days, but that she has at least been compensated in some measure. When they ask how, she brings in a man, who having been a thralled has purchased his freedom by keeping the king's accounts, and turns out to be her monk.
Thus it concludes.
Whether someone would actually read this and whether all the reading that it would take to make it halfway good would be justified are, of course, another story. But it is certainly a very evocative tale and it has brought fourth a lot of ideas from me.