The brothers Fitzgerald – simultaneously Irish Establishment and also Irish Rebellion of 1798 – and representing Ireland in the new Union Flag of 1801

The Fitzgeralds had been in Ireland since 1169, Anglo-Norman in origin, who prior to that had come to England with William the Conqueror in 1066. In Ireland, they had been Earls of Kildare from 1316 onwards, the era of the Edward and Robert The Bruce campaign, with the Fitzgeralds fighting against the Scots.

450 years later, James Fitzgerald, 20th Earl of Kildare, became the 1st Duke of Leinster in 1766.

James and his wife Emily Lennox (who was English, of Scottish and also Royal ancestry) were among Ireland's uppermost social establishment élite. Emily features in the writing of  Stella Tillyard, whose Aristocrats: Caroline, Emily, Louisa and Sarah Lennox 1740-1832 included Emily and became a BBC television series (Wikipedia here). Here's a portrait of Emily by Scottish artist Allan Ramsay:

James and Emily had 18 children – including two very contrasting boys.

• Their oldest surviving son, William Robert Fitzgerald (1749-1804), seems to have been the ultimate establishment man. William's entry in the Dictionary of Irish Biography describes him as 'somewhat pompous'. He was chosen as Grand Master of the freemasonic Grand Lodge of Ireland in 1770, and on 5 Feb 1783 he was one of the knights of the new Most Illustrious Order of St Patrick - which took upon itself his family crest, the red saltire 'Arms of Fitzgerald' as its insignia. Here he is:

• William had 'procured' the election to Parliament of his 14 years younger brother, James and Emily's fifth son, Edward Fitzgerald (1763–1798), in 1790. Two years later Edward became sympathetic to the French Revolution and also supported the new Society of United Irishmen, which sought to reform, but later to overthrow, Ireland's establishment of which his own family was such a key part. Edward eventually became a member of the United Irishmen in 1796. He was arrested in the early stages of their 1798 Rebellion, but died in Newgate Prison as a result of wounds sustained during that arrest.

(Stella Tillyard's biography of Edward Fitzgerald, entitled Citizen Lord, was published in 1998 on the 200th anniversary of his death. Edward's life story was written by Thomas Moore, and also by Patrick Byrne in his 1955 book Lord Edward Fitzgerald). Here's Edward:

• In the aftermath of the failure of the 1798 Rebellion, on 31 December 1800 the Parliament of Ireland was abolished, and Ireland was governed from a new, combined, Parliament in London from 1st January 1801. The 'Arms of Fitzgerald' were incorporated into a new flag for the new administration, to represent the inclusion of Ireland within the previous design which had visually merged the pre-existing historic flags of both England and Scotland since 1606 (designed for the 'Union of the Crowns' when King James VI of Scotland also became King James I of England).

• A red flag with a white shield and red saltire might have been used by the Dublin Volunteers in the 1700s. There is a glimpse of this in a detail of the famous Francis Wheatley painting The Dublin Volunteers on College Green, 4th November 1779, said to be a commemoration of the birthday of King William III, and gathered around the statue of William which was there at that time.

Contemporary prints of the painting include a caption 'To His Grace William Robert Fitzgerald, Duke of Leinster & of the Kingdom of Ireland...' and the Fitzgerald coat of arms. He was a Colonel in the regiment.

• Coincidentally, the family crest of the boys' mother Emily Lennox Fitzgerald (whose roots trace back to the Lennox of Woodhead clan of Dumbartonshire in Scotland) was the very same design - a red saltire, sometimes with 'engrailed' lines, and also often depicted with four roses in the white quadrants. The badge of local football team Clydebank FC is based on it, but with four different locally-relevant emblems where the roses usually appear.