The old fort at Greencastle has been extensively damaged in a mystery fire.
Gardai said they received a report of a major fire around 10pm last night.
A number of units of the fire service attended the incident and the blaze was brought under control just after 1am this morning.
A Garda spokesman said a full investigation has been launched into the cause of the blaze.
“Extensive structural damage has been caused in the overnight fire and a major investigation will take place.”