Complex sentence: St Stephen's Green is the closest point between the Green Line and the Red Line, but a proposed extension of the Green Line across the River Liffey will cross the Red Line at the junction between O'Connell Street and Abbey Street.
Split into simpler sentences: St Stephen's Green is the closest point between the Green Line and the Red Line, and an extension from the Green Line had been proposed. Plans were drawn up for an extension which would see the Green Line cross the River Liffey, and intersect with the Red Line at the junction between O'Connell Street and Abbey Street.