You can run searches on NNEREN.com for sold properties. This only will list out properties sold that were listed in the MLS.
Just hit the sold link, check off whatever towns your interested in and put "island" in the key word search and you're good to go.
http://nneren.com/listings/?Status[any][0]=SLD&Status[any][1]=UAG&Status[any][2]=SOLD&Status[any][3]=UNDER+AGREEMENT&Status[any][4]=RNT&Keyword=island&Town_State[]=Gilford+NH&s=-Date+Listed
Copy and paste that entire link into your web browser it should pull all the sold island properties in Gilford.