The name Kellan is of Irish origin, derived from the Gaelic word "caillen," meaning "slender" or "lean." It is believed to have originated in the medieval period, around the 12th or 13th century, when Irish names were becoming more popular and widespread.

One of the earliest recorded instances of the name Kellan can be found in the Annals of Ulster, a chronicle of medieval Irish history written by monks in the 15th century. The name appears in reference to a man named Kellan O'Daly, who lived in the late 12th century and was a member of an influential Irish family.

In the 16th century, the name Kellan was also mentioned in the Book of Leinster, an important collection of Irish literature and historical records. This suggests that the name had gained some popularity and recognition during this period, particularly among Irish families and clans.

Throughout history, there have been several notable individuals who bore the name Kellan. One of the most famous was Kellan O'Doherty, an Irish chieftain and military leader who lived in the late 16th and early 17th centuries. He was known for his resistance against English rule in Ireland and his leadership during the Nine Years' War (1594-1603).

Another historical figure with the name Kellan was Kellan O'Donnell, who lived in the 17th century and was a member of the powerful O'Donnell clan of Donegal, Ireland. He played a significant role in the Irish Confederate Wars and fought against the English forces of Oliver Cromwell.

In the 18th century, Kellan O'Reilly was a renowned Irish harpist and composer. He was born in 1754 and was known for his skill in traditional Irish music, contributing to the preservation of the country's cultural heritage.

Moving to the 19th century, Kellan O'Shea was an Irish immigrant to the United States who served in the Union Army during the American Civil War. Born in 1837, he fought in several major battles and was awarded the Congressional Medal of Honor for his bravery.

In the 20th century, Kellan Lutz, an American actor and model born in 1985, gained recognition for his roles in films such as the "Twilight" saga and "Immortals." His name, though not of traditional Irish origin, has helped to popularize the name Kellan in modern times.