The name Brennan has its origins in the Gaelic language, specifically in Irish Gaelic. It is derived from the Old Irish word "brennu," which means "a descendant of Bran." Bran is an ancient Celtic name that has been interpreted to mean "raven" or "crow."

The earliest recorded use of the name Brennan dates back to the 5th century AD in Ireland. It was originally a surname that eventually became adopted as a given name. The name was particularly popular among Irish families with connections to the ancient Irish kingdom of Leinster.

In Irish mythology, Bran is a prominent figure. He is featured in the Irish mythological cycle known as the "Fenian Cycle," which contains stories about the legendary hero Finn MacCool and his band of warriors called the Fianna. Bran is depicted as a powerful and wise leader.

One of the earliest recorded individuals with the name Brennan was Saint Brennan of Clonfert, who lived in the 6th century AD. He was an Irish monk and scholar who established numerous monasteries in Ireland and is credited with bringing Christianity to parts of the country.

Another notable historical figure with the name Brennan was Brennan of Armagh, who lived in the 7th century AD. He was a renowned Irish scholar and abbot who helped establish the monastic school at Armagh, which became a significant center of learning in medieval Europe.

In the 12th century, Brennan O'Rourke was a prominent Irish chieftain and military leader who fought against the Anglo-Norman invasion of Ireland. He was known for his bravery and resistance against the invading forces.

During the 16th century, Brennan Connell was an Irish poet and bard who composed numerous works in the Gaelic tradition. His poems and songs were widely appreciated and helped preserve Irish cultural heritage during a time of colonial oppression.

In more recent history, Brennan Manning (1934-2013) was an American author, theologian, and public speaker. He is best known for his influential book "The Ragamuffin Gospel," which explored the concept of God's unconditional love and grace.