Two stone souterrains, or underground structures, were uncovered approximately ten metres apart close to the recently discovered stonehenge in Lismullen, which was declared a national monument by the former Minister for the Environment, Dick Roche.

It is possible that many more underground chambers and passages dating back to the Iron Age may be uncovered, as this area is being excavated for the first time.

"This is a spectacular underground complex of chambers and connecting passages with very high-quality stonework, " said Laura Grealish of the Tarawatch campaign, who will now be guarding the area while reports are made to the National Museum and the new environment minister, John Gormley.

Before leaving office, minister Dick Roche said a "material change in circumstances" would be required to reroute the M3 motorway, which is due to go through the heritage site. Tarawatch campaigners now hope this find will be substantial enough to have the road rerouted.

"We are reporting this discovery to the minister and the National Museum this morning, " campaigner Vincent Salafia said yesterday.

"We want to know if the National Roads Authority reported the discovery to the minister or the museum, and if not, why not?" The NRA said yesterday that the find was not a unique feature.