The name Kiernan is an Irish name derived from the Gaelic word "ciar," meaning "black" or "dark." It was originally a surname that evolved from the Irish Gaelic name "Ó Ciardhubháin," meaning "descendant of the dark one." The name has been in use since at least the 8th century in Ireland and has its roots in the ancient Celtic culture.

One of the earliest recorded instances of the name Kiernan can be found in the Annals of Ulster, a chronicle of medieval Irish history written by monks in the late 15th century. The annals mention a Kiernan Ua Cuinn, who was the lord of Ive-Bairrche (a territory in present-day County Carlow, Ireland) in the year 1045.

In the 12th century, a notable figure named Kiernan O'Rourke was the Bishop of Ardagh, a diocese in County Longford, Ireland. He played a significant role in the ecclesiastical affairs of the region during his tenure from 1184 to 1214.

During the 16th century, Kiernan O'Ruairc was a prominent Irish chieftain and leader of the O'Rourke clan in County Leitrim, Ireland. He is mentioned in several historical accounts for his involvement in the Nine Years' War (1593-1603), a conflict between the Irish chiefs and the English forces of Queen Elizabeth I.

Another notable figure in history with the name Kiernan was Kiernan Dempsey, an Irish soldier who fought in the American Revolutionary War. Born in Dublin in 1753, he served as a lieutenant in the Continental Army and participated in several key battles, including the Battle of Germantown in 1777.

In the 19th century, Kiernan Fitzpatrick was a prominent Irish nationalist and Member of Parliament for County Louth in the United Kingdom of Great Britain and Ireland. He was an advocate for Irish Home Rule and played an active role in the Irish Parliamentary Party during the late 1800s.

While the name Kiernan has its roots in Ireland, it has also been adopted by other cultures and regions over time, although its origins can be traced back to the ancient Gaelic language and Celtic heritage.