This very ancient family claims royal descent, and deduces its pedigree from the celebrated Irish monarch, Brian Boru, who ascended the throne in 1002, and fell at the memorable battle of Clontarf, in 1014.
From this prince descended the Kings of Thomond; of which
TURLOGH, King of Munster and principal High King of Ireland, had, with other issue, Dermot, King of Munster, from whom descended, in 1528,
CONNOR O'BRIEN, King of Thomond, who married Anabella, youngest daughter of Ulick De Burgh, 1st Earl of Clanricarde, by whom he left four sons, in minority, at his decease, when the principality was usurped by his brother,
MURROUGH O'BRIEN, who, repairing to England by the advice of the Lord Deputy of Ireland, in 1543, surrendered his royalty to HENRY VIII, and was, in recompense, created Earl of Thomond for life, and BARON INCHIQUIN to his own heirs male.
His lordship wedded Eleanor, daughter of Sir Thomas FitzGerald, Knight, and dying in 1551, left issue,
DERMOT, his successor;His lordship was succeeded by his eldest son,
DERMOT, 2nd Baron, who espoused Margaret, daughter of Donough O'Brien, 2nd Earl of Thomond.
He died in 1557, and was succeeded by his son,
MURROUGH McDERMOT, 3rd Baron (c1550-73), who wedded Mabel, daughter of Christopher, 6th Baron Delvin, and had issue,
MURROUGH, his successor;His lordship was slain by Dermot Reagh O'Shaughnessy in 1573, and was succeeded by his son,
MURROUGH, 4th Baron (1562-97), who wedded Margaret, daughter of Sir Thomas Cusack, LORD CHANCELLOR OF IRELAND.
His lordship fell from his horse and drowned, in 1597, when fording the River Erne, near Sligo, during the Nine Years War.
He was succeeded by his son,
DERMOT, 5th Baron (1594-1624), who wedded Ellen, eldest daughter of Sir Edmund FitzJohn FitzGerald, and had issue,
Henry;His lordship was succeeded by his youngest son,
MURROUGH, of whom we treat;
Honora; Mary; Ann.
MURROUGH (1618-74), 6th Baron, who was created, in 1654, EARL OF INCHIQUN.
MURROUGH (1726-1808), 10th Baron, was created MARQUESS OF THOMOND, in 1808.
- Murrough O'Brien, 1st Marquess and 10th Baron (1726–1808)
- William O'Brien, 2nd Marquess and 11th Baron (1765–1846)
- James O'Brien, 3rd Marquess and 12th Baron (1768–1855)
Barons Inchiquin (1543; Reverted)
The heir presumptive is the present holder's second cousin Conor John Anthony O'Brien (born 1952).
- Lucius O'Brien, 13th Baron (1800–72)
- Edward Donough O'Brien, 14th Baron (1839–1900)
- Lucius William O'Brien, 15th Baron (1864–1929)
- Donough Edward Foster O'Brien, 16th Baron (1897–1968)
- Pádraig Lucius Ambrose O'Brien, 17th Baron (1900–82)
- Conor Myles John O'Brien, 18th Baron (b 1943)
According to history, the original castle on the site is said to have dated back to the 11th century, and was more rustic in nature than the existing castle of today, similar in style to Bunratty castle.
Like other castles of the times, it served as a defensive stronghold.
From the time of Morrough O’Brien (the original owner of Dromoland) until the 16th Baron Inchiquin - who still owned the castle in the 1960s - the Inchiquins lived at Dromoland for more than 500 years.
In 1736, a second castle was built in the design of the Queen Anne period with a wing enclosing a central courtyard.
This wing of the castle remains today and is almost a century older than the other sections of the castle.
The present castle was completed in 1826 by the 4th O'Brien Baronet in Gothic style, with four large towers made of a dark blue limestone that was cut from a nearby quarry, and built at great expense for the times.
The Castle is dominated by a tall, round corner tower and a square tower, both of heavily crenellated. There are also smaller towers and a turreted porch.