While the name Grange Con is said to be of Celtic origin the earliest recorded history of it is from the Middle Ages when the Abbot of Baltinglass Abbey had a castle built there. The monks are also said to have had a mill there powered by the local stream. Most of the surrounding land belonged to the Abbey at the time. Following the dissolution of the monasteries, Thomas Eustace was in 1541 made Viscount Baltinglass and granted the lands of the Abbey including the Castle and lands at Grangecon. These were forfeited in 1581 following a rebellion by the third Viscount and subsequently granted to Sir Henry Harrington. It appears that these lands remained in the Harrington family for a number of generations. The house at Grangecon Demesne later became the seat of the O'Mahony clan and remained so until about 1930 when Pierce O'Mahony, the last "the O'Mahony" died. See for more detailed information about the O'Mahony connection with Grangecon. Some ruins of an older castle still exist in the grounds but these are quite minimal. Grangecon Demesne is now home to the well-known Grangecon Stud, breeder of top grade thoroughbred racehorses.