Brian Boru, High King of Ireland (see here and here),
THE QUEEN'S IRISH ANCESTORS
While we are all re-evaluating the Queen as someone who might actually like Ireland, it might be timely to summarise her extensive Irish ancestry.
The two great Norman families of medieval Ireland were the Butlers and the Fitzgeralds. The Queen directly descends from both many times. The most recent descents I can see for her are from James Butler, 1st Duke of Ormonde (died 1688), from James Fitzgerald, 10th Earl of Desmond (died 1529) and from Gerald Fitzgerald, 11th Earl of Kildare (died 1585). Through the latter she descends from Irish heroes Garret Mor Fitzgerald, 8th Earl of Kildare, and Garret Og Fitzgerald, 9th Earl of Kildare. Her ancestor Baron Portlester lies in a spectacular tomb  in St.Audoen's church on High Street in Dublin.
In terms of Gaelic families, the Queen descends (remarkably) from the rebel Hugh O'Neill, Earl of Tyrone (died 1616) and also from Diarmaid O'Brien, 5th Baron of Inchiquin (died 1624). Through the latter she descends from Connor O'Brien, King of Thomond (died 1540) and Domnall Mor O'Brien, King of Thomond (died 1194). Ultimately, she descends from Aoife Mac Murrough (who married Strongbow in 1170) and from Brian Boru, High King of Ireland (died 1014).
In fact, Ireland is littered with her ancestral homes and her ancestors' tombs. Her most recent Irish-born ancestor was apparently Richard Wellesley, 1st Marquess Wellesley (died 1842). Given all this, it does seem unfair that she was never allowed visit her ancestral homeland until now.