I would have thought the reason for the Union Jack being refered to as the Butcher's Apron was obvious to anyone with a skeric of knowledge of Irish history. Oliver Cromwell came to Ireland in 1649 with troops and massacred thousands of civilians in Drogheda and Wexford.
After that, it got worse, battles raged and the Irish were slaughtered again. The Battle of Vinegar Hill, the Irish potato famine of 1850s wherein the British threw families out of their homes to die in the bog, estimates of about one million?
Not just men but women and children as well. Today the population is 3 million in Republic of Southern Ireland and one and a half in Northern Ireland. It was 8 million in 1840!