Markree Castle is a castle located in Collooney, County Sligo, Ireland. It is the ancestral seat of the Cooper family, partially moated by the River Unshin. Today it is a small family-run hotel.
In the 1830s the Observatory on the grounds of the Castle had the largest refracting telescope in the world. What today is known as the asteroid 9 Metis was discovered there in the 1840s by the Coopers' observatory staff.