The name means "castle at the confluence" - Caislean an Chomair in Irish, for the meeting of the Dinin and Cloghogue rivers. Wandesforde, granted the estate by Charles I's Lord Lieutenant Thomas Wentworth, recruited Yorkshire settlers for their skills in ironwork, weaving, pottery, and forestry. He laid out the streets, planted a forest on the demesne, and died in 1640 before he could see how thoroughly it would work. His descendants built two great houses in succession: the second, completed in 1802 for Lady Anne Ormonde, was a castellated Gothic-revival mansion famous for having 365 windows - one for each day of the year. By the 1970s the family had moved out. In 1975, the house was demolished. Only the main entrance, the gate lodge, and the sweeping avenue survived. The Discovery Park sits on the demesne grounds today.

The Leinster Coalfield runs from northern Kilkenny into Carlow and Laois, and Castlecomer sat in its center. Mining began in the mid-seventeenth century with iron ore; the smelting furnaces cleared huge tracts of oak forest. Then the coal seams beneath the shale were found, and the Wandesfordes opened a series of mines that defined the local economy for three centuries: the Old Three Foot Seam, the Jarrow Seam, the Skehana Seam. The coal was high-quality anthracite, low in sulphur, sold within a twenty-mile radius and carted out by horse. The working conditions were harsh. Miners formed secret societies, trade unions, even Communist organizations in the twentieth century. In 1969, after three hundred years, the last colliery closed. The trackbed of the old railway out to the mines is now a footpath through the woods. The concrete piers that carried the rails across the Dinin River still stand in the river, weathered and overgrown.

In the 1860s, a scholar named William Booking Brownrigg came to Castlecomer to look at the plant fossils preserved in the coal shales. He found something else. The miners had been bringing up the bones of something far more interesting - ancient amphibians, dating from the Carboniferous period, more than 300 million years old. Edward Percival Wright, professor of geology at Trinity College Dublin, took over the investigation and brought in Thomas Henry Huxley, Darwin's bulldog and Britain's most famous biologist. In 1867 they published On a Collection of Fossil Vertebrata from the Jarrow Colliery County Kilkenny Ireland. Huxley described at least ten genera of fossil amphibians from the site; five had never been seen before. The Jarrow assemblage is still considered one of the most important Carboniferous tetrapod finds in Europe. The fossils sit in the National Museum of Ireland and Trinity College Dublin. Replicas are at the Discovery Park, alongside life-sized reconstructions of the swamp forests where these animals once lived.

On New Year's Day, the town gathers for the Castlecomer Wellie Race. The tradition began in 1978, when men from the Powley hills above the town - playing cards on St. Stephen's Day - decided they needed to work off the Christmas excess by running a cross-country course in wellington boots. In 1981 the race went public, with a five-kilometer circuit around Kiltown. By 1982 there was a float parade. Race personalities over the years have included Olympic boxer Mick Dowling, broadcaster Micheal O Muircheartaigh, and Olympic gold medallist Michael Carruth. The event raises money for charity. It also says something about how Castlecomer made the transition out of coal. The mines closed in 1969; the Castlecomer Discovery Park opened in 2007 on 80 acres of the old Wandesforde demesne. Today there are tree-top adventure walks, a Leap of Faith jump, fallow deer in the walled garden, and craft studios in the restored stables. Tourism is the new industry. The wellies still come out on New Year's Day.