Complex sentence: The islands council areas were unitary council areas dating from 1975, created under the Local Government (Scotland) Act 1973 and notably the only unitary authorities created by the local government reforms of the 1970s in Great Britain; which generally applied a two-tier structure.
Split into simpler sentences: The islands council areas were the only unitary councils created under the Local Government (Scotland) Act 1973, which came into force in 1975. Notably, they were the only unitary authorities created by the local government reforms of the 1970s in Great Britain; which otherwise applied a two-tier structure.