They want local government to find a solution to the problem.
Three heritage sites in Offaly are among the most at-risk in the country, according to An Taisce.
The house and outbuildings at Durrow Abbey, mill buildings and industrial structures from the old sawmills in Clara, and the original stable block at Charleville Castle have all been listed as problem areas.
An Taisce's National Advocacy Manager, Ian Lumley, says they're highlighting the issue in the hope local development groups and the County Council can come together to find a solution.
He says they are three impressive 19th century landmarks, and should be maintained for heritage and safety purposes: