The house is understood to occupy the site of a post-medieval corn mill, and the possibility that an older structure preceded it on the same ground has never been ruled out.

The trail begins with a land grant recorded in the Calendar of Patent Rolls of Ireland. In 1611, Robert Nugent of Walshestown was granted a watermill in what the document calls the manor and town of Kilcowle. That placename, rendered elsewhere as Kilcooley, refers to the southern portion of the Dysart townland, and its memory survives locally in the name of Kilcooley House, situated close to the shoreline of Lough Ennell. Patent rolls of this period were a common mechanism by which the Crown formally transferred lands and rights to grantees, so Nugent's mill would have been a working economic asset, not a symbolic one. Whether the mill he received in 1611 was itself built on an earlier medieval foundation, or whether Millbrook House later rose on that same ground by coincidence, remains an open question. The physical evidence, if it ever existed in an identifiable form, has not been located.