There have been two creations of the title Baron Offaly, both in the Peerage of Ireland.
Two earlier medieval creations as Baron of Offaly existed for an earlier FitzGerald, who owned land in County Kildare, Ireland, including what was then "Offelan", a word that derived from the Irish Uí Faelain and not from what is now County Offaly. The original grant of the lordship of Offelan was in 1175 to Maurice FitzGerald, one of Strongbow's main supporters.
In 1538–50 the title was recreated for Brian O Conor Faly, who was based in neighbouring County Offaly.
The first modern creation by Queen Mary I was for Gerald FitzGerald on 13 May 1554, who was also created Earl of Kildare at the same time and later restored to the senior earldom of Kildare (forfeit by his half-brother, Thomas) in 1569. The title (as well as the second creation of the earldom), became extinct in 1599 when the 13th earl's branch of the family became extinct and the senior earldom passed to his cousin, Gerald.
The second creation of the title was for Lettice Digby on 29 July 1620. She was the daughter of Gerald FitzGerald, Lord Offally (1559–1580; the eldest son of the 11th earl, who predeceased his father) and the wife of Sir Robert Digby. Her eldest son, Robert, was created Baron Digby on the same day. The patent of the barony stipulated that it was not to pass to Lord Digby, but instead to the head of the House of Kildare, who, at the time of Baroness Offaly's death in 1658, was George FitzGerald, 16th Earl of Kildare. The 20th earl was later created Duke of Leinster in 1766 and the dukedom and barony remain united.