 1810) of Turbotstown House, Coole, County Westmeath has been attributed to Johnston.He assisted in the design of plans that were later used for St. Andrew's Church, Westland Row, Dublin, and designed the east gates of Slane Castle demesne in County Meath..In Drogheda, County Louth, he added the Gothic tower and balustrade to Saint Peter's Church of Ireland church in 1793, designed the Corn Exchange (now the Court House) in Fair Street, and designed Saint Peter's Roman Catholic Church in West Street, that was replaced by a Neo-Gothic church in 1881.