Complex sentence: The changing political atmosphere had long been signalled by the arrival in Sackville Street of further monuments, all celebrating distinctively Irish heroes, in what the historian Yvonne Whelan describes as defiance of the British Government, a "challenge in stone".
Split into simpler sentences: The changing political atmosphere had been indicated by the gradual arrival in Sackville Street of further monuments, all celebrating distinctively Irish heroes. The historian Yvonne Whelan describes this proliferation as defiance of the British Government, a "challenge in stone".