Intriguing...

I worked this up as a bridge, late in the section telling of the beginning of the curse at Darian's Point. The clau Ui Briuin and the Tautha De Danann have joined forces to fight the terror of the harpies. They have killed four of the seven, but only at a cost of a great many men. Both sides are exhausted, so Morriggan, who brought the harpies to life, brokers a truce and carries peace terms to the beleaguered creatures.

Now comes the one who gave us birth
And sorrow fills her every step Then soon her voice, with little mirth, Will say, "Agree or die." We wept. This end was seen so long ago In how she joined with men despised Once by her, but her heart will show She cares not. War will be reprised If we contend our fault was pure And did but only what she wished. "That claim," she said, "will but ensure You three no longer will exist."
"You made us whole," did we reply."Female times seven, built with wing To roam the earth and own the sky. But in each heart let sadness sing. No one for us to live and breathe In harmony. No male to join With each of us. And so we grieve As you then forbid us to coin Our depth of pain and suffering. To loneliness should we submit And hide our lives. Let each day bring New joy to worlds we must forget."
"The fault is mine," said she. "I know That I have done to you great wrong. And punishment is now the goal Meant for myself. And so, ere long I give to you one simple right So long as you will hold this pact When storms approach in dead of night Then fly you will, not be attacked. No foe approach. You will stay free To dance as waves crash to the shore And feed upon fish in the sea. And thus it shall be, evermore." 
"What pleasure comes from such a choice As to have no full life?" we cried. "And let those creatures then rejoice At how our four sisters have died?" "That is not all," our mother claimed. "Each hundred years at harvest moon A young man of my line is named And given as your feast. He soon Is readied at Point Dar Rian To seal this promise with his life Once rich and full." Thus it began. And they pronounced the end of strife. 
But unsaid in our mother’s claim Was how someday this pact would end With the last death of he whose name And blood from her line did extend. Now soon will we join with our foes To give our promise to them all. In cliffs we live so no one knows Of our existence, but to call Forth myths and tales of years gone by When king and queen of great renown Did walk the land and own the sky, Where now none more shall wear the crown.