The Morrígan is mentioned throughout the series, but who is she? Or rather, who are they?
The Morrígan is a triple goddess in Irish mythology. She represents three Tuatha De Danann women named Badb, Macha, and Neman (Rose, 2006). The Morrígan is also referred to as the Morrigu and Mor-rioghain. She has (or they have) the appearance of a gray-haired old woman or sometimes a scaldcrow. Carol Rose (2006) refers to the Morrígan as evil, though she's (they are) depicted in a better light in some other stories.
In Gods and Fighting Men, Lady Gregory (2004) the Morrígan is referred to as Badb, Macha, and the Morrigu (this may be Neman) and all three are collectively known the greatest of the women of the Tuatha de Danann. They are also referred to as queens and as goddesses of battle. Badb is directly called a battle goddess and the Morrigu is known as the Crow of Battle. They are clearly portrayed in the stories as women to fear, stirring up trouble in Ireland in the forms of quarrels and even battles. However, there is not too much detail as to why they were causing these battles. They also play central roles in building the new home of the Tuatha de Danann when they settled in Ireland and in cloaking their city from the outside world.
Rose's (2006) Spirits, fairies, leprechauns and goblins: An encyclopedia refers to one of the Morrígan women, Neman, as originally being a norse goddess. She is associated with the Valkyries, who flew over battlefields choosing heroes and causing others to die. They also ushered fallen heroes to their homes in Valhalla after death.
In The Faerie Apothecary Mysteries, the characters inspired by the Morrígan are not portrayed as evil. (On a side note here, I don't think of the characters in Moss Hill as being the actual people of mythology, but rather inspired by them.) The Morrígan-inspired characters are Tuatha de Danann, the most powerful among the Otherworld. I do associate them with battle, but more as protectors of Moss Hill. I won't go into further detail here so as not to share any spoilers, but they do play a role in the series and will continue to be mentioned in future books.
Gregory, Lady I.A. (2004) Gods and Fighting Men.Urbana, Illinois: Project Gutenberg. Retrieved October 30, 2018, from http://www.gutenberg.org/files/14465/14465-h/14465-h.htm
Rose, C. (2006). Spirits, fairies, leprechauns and goblins: An encyclopedia. New York: W. W. Norton & Company.