I agree with Beith, that's quite a task you've laid out for yourself! There might be a few ways you could pare that down a little. Since your friend is writing a poem about Derdriu of the Sorrows, perhaps you might want to list the gods who have a hand in that tale, or the tales associated with her, or with Ulster or Scotland, as part of the story takes place in each location.
You might want to look up:
Macha - sovereignty goddess of Ulster, and eponymous benefactress of Ulster's capital, Emain Macha; sometimes (mistakenly, in my opinion) described as an aspect of the Morrigan
Medb - sovereignty goddess of Connacht, who leads the Men of Ireland into Ulster to take the Donn Cuailgne (Brown Bull of Cooley); painted as the villain of the Tain, the main event to which the story of Derdriu is attached
Morrigan - (I would say sovereignty) goddess of war, battle, carnage, prophecy, testing, transformation, and a host of other things; catalyst of the Tain, a key player in the story; often described as a triple goddess with three names, although there is much confusion over these (Morrigan, Badb, Nemain, Macha, Anu, etc.)
Lugh - a god for all seasons, weathers, and occupations it seems!; father of Cu Chulainn, who abducted his mother Dechtire from Emain Macha on her wedding day
Cailleach - although not part of the Tain matrix, she is a goddess with a strong presence in both Ireland and Scotland where Derdriu's story takes place; strongly associated with the land, wisdom, testing, creation, transformation, initiation, and scads more!
There are many to choose from, that's for sure. I hope these are of some help. Good luck in your searching!