Megalithic Portal Tomb Identified in Ireland

New research looks set to answer a long-standing question about the status of a mysterious tomb-like structure uncovered in Cork Harbour many years ago.

Archaeologists have been split as to whether it was prehistoric or a more recent 19th-century “folly”.

However, Connemara-based archaeologist Michael Gibbons now says there is conclusive evidence the Carraig á Mhaistin stone structure at Rostellan on the eastern shore of Cork Harbour is a megalithic dolmen.

Mr Gibbons has also discovered a previously unrecognised cairn close to the dolmen which would have been concealed by rising sea levels, and which he is reporting to the National Monuments Service.

The Carraig á Mhaistin dolmen at Rostellan is listed by some guides as Ireland’s only inter-tidal portal tomb.

In fact, there are two such inter-tidal tombs, Mr Gibbons says.

He says doubt about Carraig á Mhaistin’s age meant it was not included in the State’s survey of megalithic tombs of Ireland conducted by Professor Ruaidhrí de Valera and Seán Ó Nualláin over 40 years ago.

“At that time, it was suggested that it could have a folly or type of ornamental structure commissioned by local gentry at the nearby Rostellan Castle estate, and dating from the 19th century,” Mr Gibbons says.

The small chamber at the tomb stands at the western end of the cairn, which is 25m long and 4.5m wide. Picture: Michael Gibbons

His recent field trip to Rostellan has thrown up additional details, including the discovery that the small chamber at the tomb stands at the western end of the cairn, which is 25m long and 4.5m wide.

This is significant, as a portal and court tombs “occasionally have intact long cairns which are both intended to provide structural support to the chamber itself, and to enhance visual presence in the landscape”, he says.

The cairn is “partially entombed in estuarine mud” and it is probable a great deal more of the structure is concealed below the surface, Mr Gibbons says in a report he has written on the monument.

He notes it is not known for certain when the area was inundated by rising sea levels, but levels at this part of the Cork Harbour shoreline are believed to have been stable for 2,000 years.

Mr Gibbons says the only other known inter-tidal portal tomb on the island is at “the Lag” on the river Ilen, between Skibbereen and Baltimore in West Cork.

Portal tombs or dolmens were often known as “Diarmuid and Gráinne’s bed”, being ᴀssociated in folklore as resting places for the fugitive couple who were pursued by Fionn mac Cumhaill, Gráinne’s husband.

Mr Gibbons says many were built close to the coast, but the two known tombs in the inter-tidal zone may have been part of a wider network which did not survive the “high energy environment” of the Atlantic seaboard.

He says recent extreme weather has destroyed Sherkin Island’s sole megalithic tomb on Slievemore townland, just 3m to 4m above the high water mark.

Mr Gibbons says the Sherkin structure had been an example of a “very fine wedge tomb”, but it was initially severely hit by the storms of 2014, which caused substantial damage to coastal archaeology in a number of locations.

What remains of the Sherkin wedge tomb has been almost entirely eroded away by recent storms, but there are some structural stones remaining which would warrant a rescue excavation, he says.