[5] The present star fort was built in 1587–88 by Queen Elizabeth I to defend Waterford from possible invasion by the Spanish Armada.

Oliver Cromwell failed to retake Duncannon in 1649, but it surrendered in 1650 after a blockade led by Henry Ireton.

The fort at Duncannon (commanded by William Fawcett) was one of the few places in County Wexford that did not fall under United Irishmen (Rebel) control during the 1798 Rebellion; the fort and town thus became a sanctuary for fleeing loyalists and troops in south Wexford and was also used as a prison and place of execution for rebels, from which it gained infamy as the Croppy Boy's site of execution in the song "The Croppy Boy."

It was occasionally used by the FCÁ (Army Reserve) for summer camps before being given to Wexford County Council in 1993 and later opening as a museum.

The landward defences had an angled redan projecting from the centre of the front, a deep ditch and a medieval wall.