A sprawling Victorian Gothic complex on Armagh Road, originally opened in 1869 as the Monaghan District Lunatic Asylum and renamed St Davnet's in 1916. At its peak it housed more than a thousand patients in long, severe blocks of grey limestone. Most of the working hospital functions have since moved out and parts of the site are being converted into a mix of community, educational, and heritage use.
It is not strictly a visitor attraction but it is one of the largest historic complexes in any Irish county town, and the scale alone tells you a lot about how Irish society treated mental illness in the 19th century. Walk the perimeter if you are interested in the architecture of institutions.