Lismore is over the Knockmealdowns in County Waterford, about forty minutes from Cahir by the Vee road. You would make the drive for the gardens. You would also make it for the drive. Together they are the best full day out from Cahir that does not involve the castle in the town.
The gardens
The gardens run to about seven acres inside the seventeenth century outer defensive walls of Lismore Castle, and they are said to be the oldest continually cultivated gardens in Ireland. That is a claim you can feel walking around them: this is not a restored garden or a recreated one, it is a garden that has simply kept going.
They are laid out over two parts, an upper walled garden that is still productive and a lower ornamental garden, with views out over the Blackwater valley and back up at the castle itself.
The Yew Walk
The Yew Tree Avenue is older than the garden around it. Edmund Spenser is said to have written part of The Faerie Queene under these trees, which may or may not be true and is entirely plausible: he held land in Munster and moved in exactly these circles. The avenue is a genuinely remarkable piece of living structure regardless of who wrote what under it.
Lismore Castle Arts
Admission to the garden includes the contemporary art gallery, Lismore Castle Arts, in the west wing of the castle. The wing had been derelict and was reworked by the Cork architect Gareth O'Callaghan into exhibition space, and the programme runs to serious Irish and international artists rather than local craft.
A very old garden and a very contemporary gallery on one ticket. Most people allow less time than the combination needs.
When to go
The gardens are seasonal and are closed over the winter, so check the opening season on the Lismore Castle Gardens site before you drive over. The castle itself is a private house and is not open, apart from the gallery wing.
The drive
Take the Vee. The R668 climbs out of the Tipperary side over the Knockmealdowns through the Vee gap and drops into the Blackwater valley. One of the best roads in Ireland. There are places to pull in at the top. Stop at one. In late summer the heather on the upper slopes turns the whole hillside purple.
Allow more time than the distance suggests, both because the road is slow and because you will want to get out of the car.
Making a day of it
Lismore itself is a small heritage town with a cathedral and a good main street, and it justifies an hour after the gardens. Coming back over the Vee in the evening light is the right way round to do it.
If you want more in the same direction, things to see covers Mitchelstown Cave, the Galtees and the rest of the country around Cahir, and things to do in Cahir covers the town itself.