The causeway to the north suggests that this was, originally, a large crannog

Notes on geophysics in the garden of Island House by H. Christie:
‘Results: Magnetic gradiometric survey was carried out by the University of Glasgow on 3 May 2019. The survey area comprised approximately 400m2 situated inside the back garden.
Notes: The results from Island House are difficult to interpret based on the small area available for survey. There are four possible linear features, which are oriented WNW by ESE and located in the northwest and eastern portions of the grid. All are on similar alignments. Another anomaly covers all but a rectilinear area 2.5 x 10m along the southwestern edge of the grid, but it is unclear whether this is related to a feature given the small area it comprises within the survey.
The linear anomalies in the eastern portion of the grid could indicate structural remains. The angular anomaly in the western portion of the grid could also indicate structural remains, possibly the remains of the earlier castle, but the area surveyed is too small to indicate features.’

See Holliday, J (2021) Longships on the Sands, pp. 514–7.