The annual event is a celebration of novelist and playwright Samuel Beckett.
The actress will read two of the writer's short prose works and some poetry at the festival next month.
Fellow actresses Miranda Richardson and Juliet Stevenson will also be appearing at the event.
They will take part in readings and talks about Beckett, who is best known for his avant-garde play Waiting For Godot.
The festival's founder, Sean Doran, said this year would be "every bit as ambitious as the last, permeating the fabric of Enniskillen and its environs in a celebration of Beckett that goes far beyond the usual treatment of his work".
Ryder is best known for her breakthrough role in the 1988 comedy hit Beetlejuice. She also portrayed mentally ill writer Susanna Kaysen in the 2001 film Girl, Interrupted.
The Happy Days Enniskillen International Beckett Festival runs from 22-26 August.