Who is the Morrigan?
What we know from myth, history, and inspiration
The Morrigan is the possible wife and definite lover of the Dagda. Her name means either "Phantom Queen" or "Great Queen" and is pronounced MOOR-reeg-an. Very few Gaelic deities are actually called deities in the old writing except for The Great Queen and The Good God. Both names are titles. The Morrigan has sisters Badb and Macha, making her a triple Goddess.
Her mother is given as Ernmas ("Iron Death") also called a "she-farmer" and a witch. We know very little about Ernmas, who is also named as the mother of other deities and dies in battle against the Fir Bolg. We don't know who the Morrigan's children are, but those who are sometimes mentioned are connected to destruction and war like their mother. Her son Mechi had three snakes in his heart that would have destroyed Ireland if he had not been killed by Mac Cecht.
The Morrigan is always associated with war and she appears to enjoy it. The Morrigan and her sisters never used weapons. They created panic and terror in the enemy. Although a battle Goddess (and much more) the Morrigan never fought on the battlefield. She magically aided the winning side, working as a Goddess of destiny.
She and her sisters magically fight off the Fir Bolg in The First Battle of Moytura (pronounced Moy Tura). Before the Second Battle of Moytura the Morrigan and the Dagda have sex over a river. This happens on Samhain. They are giants who each have a foot on the banks of a river. Because of that sexual union the Morrigan is seen as an Irish Sovereignty Goddess granting her favor to the Tuatha De Danann. Then she enters the battle magically fighting the Fomorians. Some Pagans believe by mating with the Dagda at Samhain the Morrigan makes sure that nine months later during Lughnasa the harvest will occur. Most believe she is promising her power to ensure the Tuatha De Danann win the battle.
The Morrigan is more than a war Goddess. She's also a prophetess and sorceress whose poetry either predicts the future or changes it. Her words encourage Lugh and the Tuatha De Danann to continue to fight until victory. Sovereignty Goddesses often encourage their chosen heroes to defend the land. Cu Chulainn is a good example.
The Morrigan started some of the circumstances that caused the Tain Bo Cualigne (“The Brown Bull of Cooley", pronounced toyn bow KOOL-ngya). She offered the Ulster warrior Cu Chulainn victory if he had sex with her, which he refused. Insulted, she said she would fight him as an eel, a wolf and a cow. Not only did the Morrigan make good on her threat, she also appears as a hag, a young woman, crow and raven. Because of his own actions, Cu Chulainn was already destined to die young as a great hero. The Morrigan's interference made his victories even greater, illustrating the love she had for him and the sometimes complicated relationships mortals have with deities.
There is an actual prayer to her from a man wanting more cattle on record. We don't have many prayers that mention the Gaelic deities so that's very important evidence of their worship.