St Johnstown originally belonged to Matthew Jacob, whose only daughter and heir married Richard Pennefather of New Park in 1782. Wilson refers to it as the seat of Mr. Jacob in 1786 but their second son Matthew J. Pennefather was resident at St Johnstown in 1814. In 1837 Lewis records St Johnstown Castle as "consisting of a high square tower in good preservation, [it] is the property of James Millet Esq who has a modern house in its immediate vicinity". The buildings were valued at £31+ in the early 1850s, James Millet was still the occupier holding the property from Stephen C. Moore. Buildings are still extant at the site.