In Irish mythology, there were number of women or goddesses who were the Sovereignty of Ireland. Among them were Morrigan (and her triple aspects as the goddess of war – Badb, Nemain and Macha), Eriu and her sisters Banba and Fodla. The three sisters, Eriu, Banba and Fodla were each a poetic name of Ireland. They were Sovereignty of Ireland, as well as Danann goddesses.
However, Eriu was the most famous of the three sisters. In the Lebor Gabala (Book of Invasions) and Cath Maige Tuired (Second Battle of Mag Tuired), Eriu had a lover, named Elatha, who was Fomorian king. She became the mother of King Bres of Ireland, when Nuada lost his arm. With the defeat of the Fomorians in the second battle of Mag Tuired, she was one of the wives of the hero Lugh Lamfada, as consort. When the three grandsons of Dagda murdered Lugh, Eriu married one of the brothers, named MacGreine. Her sisters married the other two brothers – Banba to MacCuill and Fodla to MacCecht. So Eriu was the mother of one king and the wife of two kings. When the Milesians arrived, the three sovereignties of Ireland knew that the Milesians would conquer Ireland, so each queen tried to persuade the Milesians to name the land after her name. Eriu, the last queen to approach the Milesians, promising them victory over her people. Eriu and her sisters fell with their husbands in the Battle of Tailtiu. As they had promised, the Milesians named the entire isle to Eriu, Erin or Eire, which is another name for Ireland. One of the most amazing goddesses was Morrigan. Morrigan was the daughter of Delbáeth and Ernmas. Morrigan also had two sisters, Badb and Macha (and possibly of a third named Nemain). Here she is seen as three separate figures. However, it is altogether possible that Badb, Macha and Nemain were all one person, known as the Morrigu, but each one represented one aspect of the goddess. So the Morrigu were the triple goddesses of war. They were also the sovereignty goddesses of Ireland, married to the high kings. Badb and Nemain had been named as the wives of Neit, a shadowy figure in Irish myths, while Macha was the wife and consort of Nuada Airgedlámh. Macha and Nuada died in the Second Battle of Mag Tuired. Macha was also said to be the wife of Nemed, the leader of the Nemedians, a race that had settled on Ireland before the arrival of the Tuatha de Danann. Before the Second Battle of Mag Tuired, the god Dagda encountered a beautiful woman at Glenn Etin at Samhain night (the eve before the battle). Dagda seduced and slept with this woman. It is believed that this woman was Morrigan, and she foretold victory to the Danann, promising aid. Each year, on Samhain night, Dagda had to mate with Morrigan, to ensure the fertility and prosperity of Ireland, because the war goddess was the sovereignty of Ireland. Sovereignty goddesses were not limited to marriage with the high king of Ireland. Each province in Ireland had a sovereignty goddess in their province. There were also another Macha, who was the sovereignty of Ulster, and in the neighbouring province, Medb (Maeve) was the sovereignty of Connacht. There are uncertainty of whether the Ulaid Macha was the same queen/goddess as the Nemedian Macha and the Danann Macha.
However, the idea of sacred marriage between a king and the goddess doesn't just appear in the Irish and Welsh myths. In fact, a king wedded to the goddess was a very ancient ritual of many different ancient cultures. And like the Celtic myths, the sacred marriage had to do with the fertility of the land. The one that come to my mind is the myth of the Sumerian goddess, named Inanna, whom the Babylonian called Ishtar. Inanna's attributes combined the Greek goddesses Aphrodite and Athena, because she was the goddess of love and war. Inanna had also being identified as the Phoenician fertility goddess Astarte, and the Egyptian Isis (Auset). So in a sense, Inanna was the sovereignty goddess of Sumer. In the Norse mythology, the sacred marriage was called hierós gámos, though the marriage was between the sky god and the earth goddess. Since agriculture was important to the Scandinavians, the union of the between the deities, would ensure the fertility of the land. The soils required not only to be fertile, but it need sunlight and rain. According to the Sumerian myths, she was the wife of Dumuzi the shepherd god. For some reason, Inanna descended into the Underworld, and Ereshkigal, the goddess of the dead, trapped her sister Inanna in her domain. However, Enki the god of wisdom send two of his creature to rescue Inanna. When Inanna escaped from her prison in the Underworld and fled to her home in the heaven, Ereshkigal send her demons after her sister. Inanna managed to protect herself and her children, but she could not protect her husband. Dumuzi was dragged into the Underworld. However part of his spirit escaped death. As sovereignty of the land, Inanna was said to be the bride of each king. Each king was seen as the incarnation of Dumuzi, the husband of Inanna. So each king actually married and mated with the priestess of Inanna (Ishtar).
Since the legend of King Arthur and the Grail had also borrowed and used Celtic motifs and symbolism, they had also used the symbolism of the sacred marriage. In the Welsh myths, Guinevere was known as Gwenhwyfar, a queen and goddess of Britain. So Gwenhwyfar was a personification of Britain; she was the sovereignty of Britain. When Arthur married Gwenhwyfar (Guinevere), he was wedded to the land (Britain). However, in the mainstream Arthurian literature, Guinevere not only representing the kingdom of Logres (Britain), but the source of Arthur's earthly power came from the Round Table. There were several versions on the origin of the Round Table, but original table (told by Wace, in the Roman de Brut, c. 1155) was constructed so that all knights were equal, with no one having precedence over the others, regardless background (see the Life of King Arthur and Origin of the Round Table). The Round Table had nothing to do with Merlin and the Grail. But as the stories of the Grail became entwined with Arthur's knight, the origin of the Round Table was changed. As early as 1200, a poet named Robert de Boron wrote a trilogy concerning the Grail: Joseph d'Arimathie, Merlin and Perceval. According to Boron, the Round Table was constructed by Merlin, using the Grail Table of Joseph of Arimathea as a model. Also Merlin made the table round because the circle was like the Earth. To shorten this story Merlin had originally built this for Uther Pendragon (Arthur's father), but at his death, King Leodegan of Camelide, the father of Guinevere, received the Round Table from Uther. When Arthur married Guinevere, Leodegan had bestowed the Round Table (and 100 knights) to Arthur as a dowry. (More detail about can be found in the legend of Excalibur, the Origin of the Round Table, and Merlin and the Grail. The whole point of this story is that Guinevere was very much the symbol of the wholeness of the Round Table and the kingdom of Logres, in some way, she represented the power of kingship more so than Arthur himself. The Queen was one with the kingdom and the fellowship of the Round Table. The health of the kingdom and the fellowship of the Round Table depended upon Guinevere, since she owned the Round Table. In the Mort Artu (Death of King Arthur, part of the romance in Vulgate Cycle), the Round Table had split because Guinevere was caught in her bedchamber with her lover Lancelot. She was to sentence to death, but Lancelot rescued her. War resulted with Arthur and his kinsmen against Lancelot and his kinsmen, and the division between two factions was symbolised the division of the Round Table. The division and war had seriously weakened Arthur's own power. However, the Round Table had further fractured when Mordred, his illegitimate son, acting as viceroy in Arthur's absence, had seized kingship and the kingdom. In this version, Mordred tried to force Guinevere into marrying him, but the queen had managed to escape. In some early versions, it was Mordred, not Lancelot, who was Guinevere's lover. Mordred in the early legend was Arthur's nephew and the brother of Gawain. The king was absence in the war against Rome, when Guinevere had willingly seduced her husband's nephew. Through marriage to the Sovereignty of Britain (Guinevere), no one could prevent Mordred becoming the king of Britain. Like the later legend, Mordred's usurpation was short-lived. In whichever versions you may have read Arthur's kingship was in crisis. By marrying his aunt, the Queen, Mordred had a legitimate claim to the throne and crown. Whoever marry the Queen, has the key to the kingdom, because the Queen was the kingdom.
In the legend of the Grail, the Grail King, sometimes also known as the Fisher King or the Maimed King, was more closely associated with the fertility of the land than Arthur. Because the Grail King was maimed, his kingdom became a desolated and barren Waste Land. (There are several version of his maiming, so I won't go over this, but if you are interested, then read the Fisher King.) Since the Grail King was wounded in the thighs and became sterile, so his land became barren. To restore the kingdom and the fertility of the land, the Grail King must be healed. Again, there are many versions of how the king was healed, but the most common version was that Grail hero, had to ask the correct question about the mystery of Grail: "Whom does the Grail serve?" The whole point of this is that land was linked to the king's health, as if he was actually wedded to the land. Cause damage or injury to the king, then the land will suffer too. As can be seen, the Grail King and his land shared a common theme of the Celtic myths. The wholeness of the kingdom depended upon the king being completely healthy. This bring us back to the Irish myths, where a king who suffered from physical imperfection or disfigurement, he was barred from kingship. Nuada lost his arm in the war against the Firbolgs. With only one arm he had to abdicate to Bres. Bres was physical beautiful and healthy, but he was unfit to rule as well, because he was a tyrant and the most ungenerous of king, which made him unpopular with his people. Such was Bres' tyranny that Nuada was first given a silver arm, so that Nuada can rule again. Later, Miach, the son of Dian Cécht, restored Nuada's arm, so that there was no uncertainty of Nuada's right to rule Ireland. Another famous king that was disqualified from ruling Ireland was Cormac Mac Airt was disfigured. Cormac, the high king of Ireland, lost one eye, so he had to abdicate to his son Cairbre Lifechair.