The name Donel is of Celtic origin, specifically derived from the Irish language. It is a masculine given name that emerged during the Middle Ages, around the 9th to 10th centuries. The name is closely related to the Irish word "donn," which means "brown" or "dark-haired."

In ancient Irish mythology, there are references to a legendary figure named Donel Dána, who was said to be a skilled poet and scholar. This association with poetic and scholarly pursuits may have contributed to the name's popularity among the learned classes of medieval Irish society.

One of the earliest recorded instances of the name Donel can be found in the Annals of Ulster, a chronicle of medieval Irish history. The entry from the year 1048 mentions a Donel Ua Lochlainn, who was the King of Ailech, a medieval Irish kingdom located in what is now County Donegal.

In the 12th century, a notable figure named Donel O'Brien was the King of Thomond, a historic kingdom in present-day County Clare, Ireland. He is remembered for his efforts in preserving Irish culture and promoting the arts during his reign.

During the 13th century, Donel Oge O'Donnell was a prominent Irish chieftain and military leader from the O'Donnell dynasty of Tyrconnell (now County Donegal). He played a significant role in the Irish campaigns against the Anglo-Norman invasion.

In the 16th century, Donel Ballagh O'Cahan was a notable Irish chieftain and Lord of Hy Tuirtre (now County Antrim). He is remembered for his resistance against the English Crown's efforts to extend control over Ulster.

It is worth noting that while the name Donel has its roots in Ireland, it has also been used in other parts of the Celtic world, such as Scotland and Wales, where similar variants like Donald and Donal have been prevalent.