The name Declan has its origins in the Irish Gaelic language and culture. It is derived from the Old Irish name Declán, which itself comes from the Latin name Decius or Decimus, meaning "tenth" or "tenth-born". The name was particularly popular in ancient Ireland, where it was sometimes given to a tenth child.

In Irish legend, one of the earliest and most famous bearers of the name was Saint Declan, who lived in the 5th century AD. He was a pioneer in spreading Christianity in Ireland and is the patron saint of the Diocese of Ardmore. Saint Declan's life and deeds are recorded in the ancient Irish manuscript known as the "Book of Lismore".

Another notable early figure with the name Declan was the Irish king Declan mac Crimthainn, who ruled over the kingdom of Mide in the 7th century AD. He is mentioned in several Irish annals and historical records from the time.

In the 9th century, Declan of Inishcaltra was an Irish monk and scribe who is credited with writing the "Annals of Inishcaltra", an important historical chronicle of events in Ireland during his lifetime.

Moving into more modern times, Declan Buckingham (1592-1636) was an English Jesuit priest who was executed for his Catholic faith during the reign of King Charles I.

In the 19th century, Declan McManus (1849-1899) was an Irish-American sculptor and artist who is best known for his statues and monuments found in various cities across the United States.

The name Declan has continued to be used throughout history, with other notable bearers including Declan Costello (1905-1964), an Irish politician and lawyer who served as Attorney General of Ireland, and Declan Mulholland (1957-1976), a member of the Provisional Irish Republican Army who was killed during a skirmish with the British Army.