Boretree Island is tiny, opposite Mount Stewart 1 acre, 2 roods at high medium tide with a high point 22 ft above sea level, surrounding mud flats exposed at low tides but still need a boat to get there. Wading bird sanctuary now. There is a ruined building on it on satellite views but the old 6" maps don't show on PRONI viewer. 7 people living there in a 2 roomed stone house with slate roof & 4 windows to the front with stable, calf house, piggery and foul house. Fresh water would have been a problem.
https://maps.app.goo.gl/KRj3AgT1QQhCYTVWAMid Island where they were 1911 seems much more sensible, 15 acres with a connecting causeway to shore except at highest tides and to South Island with more land. Buildings shown on Mid Island from 1st edition 1830's too now
https://maps.app.goo.gl/AWrX4ScMks1MbFuKA Both in Greyabbey civil parish.
https://census.nationalarchives.ie/pages/1911/Down/Grey_Abbey/Mid_Island__in_Strangford_Lough/Pretty confident the Enumerator has just put the wrong name in for the island and 1901 should read Mid Island also, one window less but same stable, calf house, piggery etc, so he didn't need a rowing boat. 17 Greyabbey Townlands enumerated both 1901 & 1911. The ruin on Boretree a shed for oyster bed work etc