Ballycowan Castle is a well-preserved ruined tower house about 4 kilometres west of Tullamore, reached by a short track off the R420 road. Built in the early 17th century by the O'Molloy family, it is a good example of the defensive tower houses built by Gaelic Irish families in the late medieval period. The walls are largely intact to their full height, giving a clear impression of the original structure. There is no formal visitor infrastructure — you approach across a field — which gives it an unmanaged quality that many more famous ruins lack.
Combined with a visit to Charleville Castle, it makes for an interesting afternoon exploring the area's medieval and early modern history. The landscape between the two is typical Irish Midlands: flat, green, and quiet.