The surname O’Gara is an anglicized version of the Irish Ó Gadhra, meaning Descendent of Gadhra, a personal name that in turn derives from the word ‘gadhar’ meaning hound or mastiff. The family originally occupied an area known as Luighne, on the borders of what today are counties Mayo and Sligo. However, by the late 13th century they had been driven out of this part of the country by the MacSurtains (otherwise Jordans) and MacCostelloes, and so moved to what became known as the barony of Coolavin, County Sligo, where they remained in power until the late 16th century. Here, close to the north-west shores of Lough Gara (Loch Uí Ghadhra), they would build a large castle known as Moygara.
In its present form, Moygara Castle consists of a large square bawn, each side measuring 51 metres, with a slightly angled residential tower in every corner and the remains of a gatehouse on the western side. The tower in the south-west corner is three storeys high, whereas those in the other corners rose just two storeys. Investigation of the site by the Moygara Castle Research & Conservation Project suggests originally a tower house stood here prior to the construction of the castle at some period between the early 16th to early 17th centuries. In 1538 Manus O’Donnell, after capturing Sligo Castle marched on and took the castle at Moygara, his son being killed by a shot fired from within the building. More than four decades later, in 1581, Moygara Castle was again attacked, this time by a number of Scots mercenaries in the hire of Sir Nicholas Malby, Lord President of Connaught. They burnt the castle and killed the son of Cian O’Gara.
By the 17th century, Moygara and its surrounding territories belonged to Fearghal Ó Gadhra who is believed to have attended Trinity College Dublin. Ó Gadhra is remembered for being the patron of The Annals of the Four Masters, a history of the country compiled by a number of Franciscan friars led by Mícheál Ó Cléirigh during the 1630s. There is some dispute over why Ó Gadhra should have assumed this position: it may be that while an undergraduate he had come into contact with antiquarian scholars James Ussher and James Ware and this inspired an interest in Irish history. Alternatively it may have come about because when young Ó Gadhra had been a ward of Sir Theobald Dillon, first Viscount Dillon who had close associations with the Franciscan order. In the aftermath of the Confederate Wars, the family’s lands were confiscated and it may be around that time that Moygara Castle began to fall into ruin. The later years of Ó Gadhra are unclear, although it is thought he was still alive at the time of the Restoration of 1660. One of his grandsons was Oliver O’Gara, an ardent Jacobite who, after the Treaty of Limerick, went into exile and joined the Irish Brigade in the French army.