Archaeologists are to dig out a portal tomb in Northern Ireland for the first time in 50 years.

The collapse of Tirnony Dolmen near Maghera has produced a rare opportunity to discover what lies beneath — and exactly how old it is.

Normally portal tombs, which are among the oldest built structures still standing in Northern Ireland, are off limits to excavators and must be preserved.

But after the massive capstone of this portal tomb fell to the ground earlier this year, archaeologists will be able to uncover the secrets it has held for millennia before repairs are carried out.

Tirnony Dolmen is between 5,000 and 6,000 years old, according to Northern Ireland Environment Agency (NIIEA) archaeologist Paul Logue.

“After standing in Northern Ireland weather for over 5,000 years some of the tomb’s structural stones have begun to crack, causing the capstone to slip,” he said.

“Before we start to repair the tomb we will excavate it to ensure that the archaeological material associated with it is recorded ahead of restoration work.

“When the tomb was first built it would have been used for interring the bones of selected members of the local stone age community. This could have included men and women, young and old. Finds from inside similar tombs include pottery and flint tools, possibly left as grave goods for use by the dead in the afterlife.

“We hope to find out more about how this tomb was built, when it was built and how it was used.”

Members of the public are invited to come along on Friday afternoons to find out for themselves what has been unearthed. The excavation will also be charted in a blog revealing the latest finds.

Mr Logue said the tomb was originally built by digging out a trench where the upright stones were embedded, packing round these with smaller stones before installing the massive capstone on top of them.

In recent years, the capstone, which weighs between two and a half and three tonnes, had begun to rock, putting pressure on the supporting stones beneath. These then moved, causing the capstone to slide further.

Such portal tombs are only found in Ireland, Wales and Cornwall, so there has long been a debate over where they originated and which portal tombs are the oldest. Archaeologists are hoping to carbon date any items they find which have fallen among the packed stones, giving an accurate date for the building o the tomb.

“We have to work out when these portal tombs appeared in Ireland. We haven’t done a dig on one of these for almost 50 years — it’s only on very very rare occasions that we decide to excavate because for something that important our job is to preserve rather than to excavate,” Mr Logue said.

“Because this one has been damaged, we can maybe answer a lot of questions.”

”When these things started getting built, there were also big rectangular houses starting to get built. We’re hoping to date the tomb and house types together - there might have been a wave of people coming into Northern Ireland with new ideas and new architecture.

“For the first time we would have seen domestic animals in Northern Ireland, for the first time we’d have seen fields. People were domesticating pigs and wild goats which became the sheep we have today.”

The excavation is being conducted by the Centre for Archaeological Fieldwork at QUB with NIEA. The general public can visit on Fridays (2pm-4pm) or follow progress via the archaeologist’s blog on www.ni-environment.gov.uk

Background

The capstone of the Tirnony Dolmen is supported by three of six upright stones, two of which form the portal.

It is believed to be a 5,000 to 6000-year-old megalithic burial tomb. One of the small non-supporting stones was badly damaged when the capstone on the dolmen fell off at the end of April.

The dolmen is remarkable for the free-standing orthostat, 1.8m high, which is beside one of the portal-stones. Behind the tilted capstone is a well-defined square chamber.