Historical secrets locked within the walls of Ireland's best preserved Anglo Norman castle could be uncovered through a new archaeological excavation.

Experts started work at Carrickfergus Castle today in a bid to find out more about the 800-year-old fortification on the shores of Belfast Lough.

Built in 1177 by Anglo Norman knight John de Courcy soon after his invasion of Ulster, the castle lies on the stretch of coastline where King William III landed in Ireland before the Battle of the Boyne in 1690.

Its storied history includes sieges by King John in 1210 and Edward Bruce in 1315 and capture by the French under Captain Francois Thurot in 1760.

The castle was used by the British Army until 1928 and it housed air-raid shelters during the Second World War.

The test excavations are part of the ongoing work by the Department of the Environment to uncover more of the castle's history and to inform future development of the landmark.

Late last year Environment Minister Mark H Durkan announced that the dungeons are to be opened up as part of a major renovation project.

The restoration work will also see the roof of the Great Tower replaced and the opening up of the ammunitions room.

The construction will allow expanded public tours of the building, as well as enabling the provision of banqueting and educational facilities.

The archaeological excavation will be conducted before the new building work starts.

It will focus on two locations, one being the Great Hall, to find out more about the date and survival of archaeology.

"This is an exciting new phase in the life of Carrickfergus Castle," said Mr Durkan.

"We do not know yet what we will find in the excavations and we want to make sure that any new discoveries become part of the visitor experience at the site."

The work at the castle is part of a £4 million investment in a number of historical and heritage sites across Northern Ireland.

Dundrum Castle in Co Down is getting a new visitor facility to present its recently excavated features while access is being improved around the ancient inauguration site of the O'Neill chieftains at Tullahogue Fort in Co Tyrone.

In the Faughan Valley, near Londonderry, 21.5 hectares of grassland is being acquired to build 17 kilometres of public pathways through the countryside setting.

Carrickfergus Borough's mayor, alderman Billy Ashe, welcomed the archaeologists.

"This is a notable development and I look forward to witnessing the excavations at first hand," he said.

"For visitors to the castle during the course of the next three weeks, the opportunity to view a live dig is an exciting proposition and will undoubtedly enhance the visitor experience."

Although the excavations will be fenced off for safety purposes, visitors to Carrickfergus Castle will still be able to view the excavations as they take place and see what the archaeologists are uncovering.

The Centre for Archaeological Fieldwork, Queen's University Belfast, will carry out the work on behalf of the Northern Ireland Environment Agency.

Online Editors