Killashee is steeped in history. From the introduction of Christianity to Ireland to the present day, Killashee has been home to many interesting events and people.

One such event relates to the founding of the church at Killashee, located on adjoining land to the hotel, by St Patrick in the fifth century. Indeed, Killashee (alias Killosy or Kilussy) derives its name from St Auxillius, an early missionary to Ireland and possibly a nephew of St Patrick, who completed and governed the church at Killashee until his death in 459.

Throughout the seventeenth and eighteenth centuries, Killashee was home to the Belling and Graydon families. Alexander Graydon settled in Killashee in 1711, when he leased the lands from Sir Richard Belling. He was sovereign of Naas in 1730 and probably built the original Georgian house at Killashee.

The Moore family, descendants of the earls of Drogheda, commissioned Thomas Turner to design a country house for them in 1861 and this is the building in which the hotel is situated. Killashee House Hotel was essentially a Victorian Jacobean hunting lodge. It was originally set on four hundred acres of farmland.

The Moore family settled well into the life of country squires and although they felt the ill effects of the famine, such a sizable estate generated a lot of employment and prosperity. At the turn of the century the estate was thriving under the owner Judge Moore. Following his death, Lt Colonel Moore inherited the estate and it continued to thrive.

In 1921 when the Lt Colonel died, his son Richard inherited Killashee and subsequently sold it to the La Sainte Union Nuns in 1927. The La Sainte Union Nuns started a preparatory school for boys. During the 1930s the Land commission divided the estate and left the Nuns with 85 acres.