In Celtic mythology, the Morrighan is known as a goddess of battle and war. However, there's a bit more to her than this. Also referred to as Morrígu, Morríghan, or Mor-Ríoghain, she is called the "washer at the ford," because if a warrior saw her washing his armor in the stream, it meant he was to die that day. She is the goddess who determines whether or not you walk off the field of battle, or are carried off upon your shield. In later Irish folklore, this role would be delegated to the bain sidhe, who foresaw the death of members of a specific family or clan.
The Morrighan often appears in the form of a crow or raven, or is seen accompanied by a group of them. In the stories of the Ulster cycle, she is shown as a cow and a wolf as well. The connection with these two animals suggest that in some areas, she may have been connected to fertility and land.
In some legends, the Morrighan is considered a triune, or triple goddess, but there are a lot of inconsistencies to this. She often appears as a sister to the Badb and Macha. In some Neopagan traditions, she is portrayed in her role as destroyer, representing the Crone aspect of the Maiden/Mother/Crone cycle, but this seems to be incorrect when one looks at her original Irish history. Some scholars point out that war specifically is not a primary aspect of the Morrighan, and that her connection to cattle presents her as a goddess of sovereignty. The theory is that she can be seen as a deity who guides or protects a king.
In modern literature, there has been some linking of the Morrighan to the character of Morgan Le Fay in the Arthurian legend. It appears, though, that this is more fanciful thinking than anything else. Although Morgan le Fay appears in the Vita Merlini in the twelfth century, a narrative of the life of Merlin by Geoffrey of Monmouth, it's unlikely that there's a connection to the Morrighan. Scholars point out that the name "Morgan" is Welsh, and derived from root words connected to the sea. "Morrighan" is Irish, and is rooted in words that are associated with "terror" or "greatness." In other words, the names sound similar, but the relationship ends there.
There's an excellent page with plenty of scholarly information on the Morrighan from Reverend Gwynarion Elessacar at http://www.elessacar.com/the_morrighan.php.