You do not "summon" a Deity, that would be just plain rude and would likely annoy the Deity in question. You might "invite" the Deity to be present or request their presence, but summoning one would be a very, very bad idea.
That is particularly true with the Morrighan. She can be a very uncomfortable Deity to work with and the lessons she teaches can be painful. I'm suspecting that you know very little about the Morrighan right now. Perhaps you should do some more research about her. In particular I'd suggest reading the story of the Morrighan and Cu Cuchulain to see how she reacts if not treated with utmost respect.