The given name Deklan is a relatively recent invention, having no clear linguistic or cultural origin. It appears to be a modern variant of the Irish name Declan, which derived from the Gaelic words "decc" meaning "whole, perfect" and "lan" meaning "full, complete."

The earliest recorded instance of the name Declan dates back to the 5th century AD when Saint Declan, also known as Deoclán or Declán, was a prominent Irish saint and one of the first bishops of Ardmore in County Waterford, Ireland. He is credited with establishing a monastic school and church in Ardmore, which became an important center of learning in the region.

Throughout the Middle Ages, the name Declan was primarily used in Ireland and parts of Scotland influenced by Irish culture. It experienced a resurgence in popularity in the late 20th century, leading to the emergence of the variant spelling Deklan.

Notable historical figures named Declan or Deklan include:

1. Saint Declan (5th century AD), the Irish saint and early bishop of Ardmore.
2. Declan Kidney (born 1959), former Irish rugby union player and coach.
3. Declan McManus (born 1954), Scottish singer and songwriter better known by his stage name Elvis Costello.
4. Declan Masterson (1938-2020), Irish actor best known for his roles in films like The Commitments and Braveheart.
5. Declan Donnellan (born 1963), English theater director and co-founder of the Cheek by Jowl theater company.

While the variant spelling Deklan is a more recent invention, it draws upon the historical roots of the Irish name Declan, which has been in use for over 1,500 years and is associated with the early Christian history of Ireland.