Fenagh Abbey, located in Fenagh, County Leitrim, is one of the oldest monastic sites in the country.

The Abbey, founded by St Caillin in the 5th century, was renowned for its monastic school which attracted students from all over Europe.

The abbey ruins date from the 15th century. Notable features include an east window of unusual design and a relief-carved 17th-century penal cross. It was pillaged by Cromwellian soldiers in 1652 and damaged by cannon fire during the Williamite wars in 1690. The last service was held in 1729.

Fenagh Abbey was later used as a place of worship for the Church of Ireland until the present church was built nearby circa 1798.

The Book of Fenagh, completed in 1516, contains verse and prose about the life of St. Caillin. A copy is now kept at the Royal Irish Academy.

Fenagh Visitor Centre houses a small exhibition about the abbey and the significant archaeological sites in the surrounding fields.