You can find an ancestor yourself, using the books listed below to pursue your lineage:
Douglas Richardson's 5 volume set "Royal Ancestry", 3 volume set "Plantagenet Ancestry", and 4 volume set "Magna Carta Ancestry", Frederick Lewis Weis' "Ancestral Roots of Certain American Colonists Who Came to America before 1700" 8th Edition (prior versions were "60 Colonists"), Gary Boyd Roberts' "Royal Descents of 900 Immigrants who came to the American Colonies, Quebec, or the United States" (prior version was "600 Immigrants") and George Edward Cocayne's "The Complete Peerage", edited by the Hon. Vicary Gibbs.
The founding Genealogist has prepared charts of known descendants of Brian Boru.
Head of Brian Boru, Dublin Castle