Two genuinely good attractions, with a river running between them. That fills a day, comfortably more if you walk. Be clear about what the town is, though: a heritage stop that repays a day, not a base that needs a week.
The two big ones
Cahir Castle sits on an island in the middle of the town and is one of the largest and most complete medieval castles in Ireland. It is complete because it surrendered twice instead of resisting, which is a better story than most castles have. Take the guided tour.
The Swiss Cottage is two kilometres south along the river, an ornamental cottage built around 1810 for the same family, its design attributed to John Nash. It is not a house, it is a Regency fantasy of one, and the interior is far richer than the thatch outside suggests.
Both are Office of Public Works sites and the Swiss Cottage is guided tour only, so check the season before you plan around it.
The walk between them
The Coronation Walk links the two along the bank of the Suir. Two kilometres each way, flat, surfaced and shaded by woodland planted in the 1790s. It is the single best hour and a half in Cahir and it costs nothing.
The Butler Trail
The Butler Trail is a self guided heritage route linking Cahir with Clonmel, Carrick-on-Suir, Fethard and Roscrea, following the Butlers of Ormond who were the dominant family in Tipperary for five centuries. There is a free audio guide app covering the route in English, French and German. If you are interested in why the castle, the cottage and the hotel on the Square are all the same story, this is the thing that joins them up.
On the water
The Suir Blueway starts at Cahir, and it is worth understanding what that means. From Cahir it is a 53km paddling trail down to Carrick-on-Suir. The waymarked walking and cycling section of the Blueway is further downstream, between Clonmel and Carrick, a 21km towpath route. The only walkable Blueway stretch at the Cahir end is the 4km castle to Swiss Cottage path.
For getting on the water, Suir Valley Adventures run guided kayaking and canoe trips on this stretch of the Suir with instructors registered with Canoeing Ireland, and take everything from a couple of hours to a full day. Book ahead. Turning up will not work.
Fishing on the River Suir covers the other reason people come to this water.
Walking and the hills
The Galtees rise behind the town and are the highest inland range in Ireland. Scaragh Wood, thirty minutes from the Square on foot, has three Coillte loops from 3.8km to 6.2km with a portal tomb on the route, and Galtymore is a three to three and a half hour hill day for anyone equipped for it. Walking near Cahir has all of it, and cycling routes near Cahir covers the bike.
With children
Cahir Park playground is the reliable answer when the castle has run its course. The riverside walk takes a buggy. The castle suits children old enough to manage stairs, because it is a real building to climb and not a corridor of information boards.
Further afield
Within half an hour there is the Rock of Cashel, the Vee drive over the Knockmealdowns, the Glen of Aherlow and Lismore Castle Gardens over the county line. Things to see covers the wider area properly.
Eating and staying
The best places to eat in Cahir, cafes in Cahir and pubs in Cahir cover the food and drink. Hotels in Cahir, bed and breakfast in Cahir and self catering in Cahir cover the beds. Getting to Cahir covers arriving.