There is no exact date for the construction of Hibernia Mansion until the 1903 ballroom addition, but rehabilitation activity has revealed that the western part of the west wing was the earlier 18th century house, an 18' x 24' stone building that was either 1.5 or 2 stories high. Along with a later 1.5 story 18' x 27' addition to the east side, this was the Ironmaster's home by 1798. A walk-in fireplace was built on the 1st floor, but it was replaced by a staircase leading to the servants' quarters. The support arch of the fireplace still exists in the cellar. This section became part of the west wing, the working portion of the house.

The now central "big house", 45' x 43' was probably built soon after Charles Brooke bought the property in 1821. Its north side suggests what the exterior of the building would have looked like in the Brooke period.

The floor plan included a longitudinal front hall with two large rooms on the north side; 18' x 20', high 9'10" ceilings; and a central chimney serving two fireplaces on each floor. The woodwork has a circle motif carved in different positions in every mantel design.
Although the era in which the rear servant's wing was added to the mansion is unknown, it is shown as being a part of the Costigan Mansion on a map of the property from the 1880's as were the nearby stables.