Icaria Prime was a planet in or near Federation space, and home to the Icarians. The planet was populated by a race called the Gretharans until the late 21st century, at which time they were wiped out by the neighboring Letheans.
In 2377, Icaria Prime was the site of an archaeological mission led by Professor Gabriel Collins, which had uncovered evidence indicating Gretharan settlement on the planet long before previously known, prior to humanity's achievement of spaceflight. Also uncovered was technology which brought painful and repressed memories in humanoids to the forefront of their minds, also causing severe physiological effects. (CoE eBooks: Remembrance of Things Past, Book I, Remembrance of Things Past, Book II)