Complex sentence: Except during a brief period in WWII it has never been necessary for Irish or British citizens to produce a passport to cross the border, during the 1970s troubles, security forces regularly asked travellers for identification.
Split into simpler sentences: Except during a brief period in WWII it has never been necessary for Irish or British citizens to produce a passport to cross the border. However during the 1970s troubles, security forces regularly asked travellers for identification.