This Irish epic can be compared to Beowulf and El Cid in terms of both epicness and oldness (and the unknown-ness of who first wrote it). It comes from the Ulster Cycle of Irish stories. The Morrígan is a somewhat minor character, but she plays a very active role. She warns the red bull of Queen Medb's (Maeve's) plan to kidnap it; and this is also the infamous story where the Morrígan tries—and fails—to seduce Cúchulainn. Drama ensues.
These cycles are not so much traditionally published works as they are groups of stories with similar themes. As far as stories concerning the Morrígan, the Mythological cycle (which we admit has a confusing name) has lots of descriptions of the Tuatha Dé Danann (The Children of Danu) as well as the stories of the two battles of Mag Tuired (see the Morrígan's Wall for more on this). The Ulster Cycle includes Táin Bó Cúailnge. Though these stories are all very old, the oldest existing collections of them date back to the 12th century.
This book by Pat O'Shea is a highly-rated fantasy novel for children, with many, many characters from Irish mythology in it. The Morrígan is the antagonist in this book—don't mess with her.
A Capcom 2D fighting game for CPS II ('90s retro right there) and later for Playstation had a playable character based on the Morrígan. This version of the Phantom Queen wasn't evil, but she was a succubus born in Scotland in the 1600s. Capcom obviously took some liberties with her origin story. The games was made into comics, and they tried to make it into a TV show in 1995, but it only lasted a few (terrible) episodes.
Christopher Moore's funny novel is about an unsuspecting guy who is saddled with the job of collecting the souls of the dead. The Morrígan, along with her sisters, is yet again the villain.
The Morrígan's appearances in the DC universe are in her triplicate form. Apparently, the goal of the Morrígan is to destroy every Amazon. Some weird, cultural crossover there.
Mike Mignola knows his mythology, and Hellboy is the best comic for all our fellow myth-nerds out there. The Morrígan is terrifying and powerful in Mignola's work, but not in a gimmicky way. We approve.
Occasionally making an appearance in Marvel comics, too, the Morrígan was most recently in a plot arc of X-Factor. She is the villain. No one is surprised.