Barnabas married Mary, youngest daughter of Sir George Fermor and widow of James, lord Sanquhar, by whom he had one son, Henry O'Brien, 7th Earl of Thomond, his successor (1621–1691); and one daughter, Penelope, who married Henry Mordaunt, 2nd Earl of Peterborough.
Henry VIII of England | Henry VIII | Henry Kissinger | Peterborough | Henry Wadsworth Longfellow | Henry II of England | Henry II | Henry III of England | Henry IV of France | Henry IV | Henry | Henry Ford | Henry James | Henry VII of England | Henry III | James Earl Jones | Prince Edward, Earl of Wessex | Louis Mountbatten, 1st Earl Mountbatten of Burma | Henry Moore | Henry Miller | Henry I of England | Henry Clay | Herbert Kitchener, 1st Earl Kitchener | Henry IV of England | Patrick Henry | Henry Mancini | Henry V | Henry David Thoreau | Joseph Henry Blackburne | Henry V of England |
The fifth baron was created Earl of Peterborough in 1628 and the title then passed to his son, the second earl, in 1644.
By his wife Elizabeth, daughter of William Howard, 3rd Baron Howard of Effingham, he left two sons: Henry Mordaunt, 2nd Earl of Peterborough and John, afterwards Lord Mordaunt of Reigate and Viscount Mordaunt of Avalon; and a daughter, Elizabeth, who married Thomas, son and heir to Edward Howard, 1st Lord Howard of Escrick.