The new trail leading to Ireland's mythical 'Gate to Hell'

Rathcroghan is one of Europe's oldest unexcavated royal sites. Now, a brand-new walking path is giving travellers access to ancient landmarks – including the cave that inspired Halloween.

From the top of Rathcroghan Mound in Ireland's County Roscommon, the view stretches across grassy fields, stone walls and grazing cattle and sheep. While this may appear as unassuming farmland, a closer look reveals dozens of large, circular earthworks and ancient burial mounds.

That's because Rathcroghan (meaning "the fort of Cruachan" in Irish) is one of the oldest and largest unexcavated royal sites in Europe. This was the ancient capital of Ireland's Connacht province and 266 archaeological relics are spread across the 6.5sq-km site – including Iron Age ringforts, Ogham stones carved with ancient Irish script and holy wells.

But one of Rathcroghan's most mysterious features is a passageway that extends deep into the earth and crosses the mythological veil into the Celtic Otherworld. Known as Ireland's "Gate to Hell", it is considered to be the birthplace of ancient traditions that evolved into Halloween.

"It's a fascinating place – not many sites can claim an entrance to the Otherworld," says John Waddell, emeritus professor of archaeology at the University of Galway.

Since many of Rathcroghan's monuments are located on private farmland, visitors have historically only been able to access some of its sites by car. But a brand-new 13km (8-mile) Rathcroghan Archaeological Walking Trail now links the complex's main highlights on a looped walking path, giving travellers new access to the land and a different perspective on how these ancient monuments were connected.

Rathcroghan's history stretches back 5,500 years. It was an important hub for ceremonial assemblies, as well asfeasts, seasonal celebrations and royal inaugurations.

The site has inspired a wealth of folklore. This is where Ireland's national epic, The Táin – Táin Bó Cúailnge ("The Cattle Raid of Cooley") begins and ends. Considered Ireland's version of The Iliad, it recalls a great raid started by Queen Medb of Connacht, an Iron Age warrior who ruled from her nearby palace.

"[Rathcroghan is] an ancient royal site, we're not talking about a palace being stuck out on the landscape; we're talking about a site where people came to gather for various reasons – to mark a celebration, the cross-quarter festivals, a marriage, a contract, a death, a funeral, crowning a king," says Rathcroghan tour guide and research assistant Elaine Conroy.

It's also sacred ground. The complex is considered one of the three chief burial places in ancient Ireland where the nation's early kings and chiefs were laid to rest, and today it's home to 40 identified burial mounds.

Roughly 1km south-west of the main Rathcroghan mound lies Oweynagat cave – believed to have been the home of the Mórrígan, the supernatural goddess associated with the birth of Halloween. The deity appears as a shape-shifter in The Táin and is associated with the festival of Samhain, which marked the coming of winter. This period – which started on 31 October – was a time when the veil between this world and the Otherworld was thinnest, and the Mórrígan and her malevolent creatures ventured from the cave. As a result, people began dressing in disguise so the spirits did not take them.

It typically takes three hours to walk the full trail. I started my day at the Rathcroghan Visitor Centre in the village of Tulsk, before then making the 4km drive to the trailhead at Rathcroghan Mound, where the new walking trail starts and ends, and set out towards the "Gate to Hell".

Rathcroghan Mound is the site's largest and most accessible landmark, an 88m-wide-by 5.5m-high (289ft-by-18ft) earthwork that has never been excavated. Yet, foundations discovered during geophysical surveys suggest that it may have been used for various ceremonies, such as the inauguration of kings.

"This was the centre of Connacht," says Mike McCarthy, an archaeologist and researcher at the Rathcroghan Visitor and Heritage Centre, as we stand at the new trailhead beside Rathcroghan Mound. "We are walking through a landscape of assembly, ceremony and inauguration, and we're in the middle of a very high-status prehistoric graveyard – burial place of the Kings of Connacht. So there is not only archaeology and history, but there's folklore and mythology and literature associated with the site."

Waddell, who conducted field research at Rathcroghan, says one of the reasons the landscape is so interesting is that it is saturated with ancient meaning.

"The geophysical surveys of [Rathcroghan Mound] have demonstrated that it had a very complicated history, with all sorts of circular structures buried in it and sometimes erected on top of it. It was a great ceremonial site, probably a great circular timber hall," he says. "That's what makes it so special – the myth and legend as well as archaeology, all embraced in the tales to tell."

From the mound, I walk with McCarthy towards one of many new stiles whose steps lead over a stone wall to connect the new trail to previously off-limits farmland. After passing an unnamed burial mound, we soon come to Relignaree, a massive 110m (361ft) circular enclosure known as "The Graveyard of Kings". A newly installed sign reveals that this wasn't just a royal cemetery – it may have also been an "elite residence", complete with ringforts and underground passages.

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Cattle have grazed this land for thousands of years, and continue to do so today. On our walk, we meet a group of cattle with calves, and McCarthy says hikers might meet livestock and curious sheep along the trail.

As McCarthy points out, the animals keep the area grazed. "The monuments are here because of farming," he says. "Otherwise nature would have taken them back." Farmers have even installed large scratching brushes for the cows so they don't use the archaeological monuments instead.

We soon come to a row of trees and scrub, and here, at the base of the trees, is an ordinary-looking ditch leading to the mythical Gate to Hell. Located 7m (23ft) underground and stretching 37m (121ft) long, this limestone shaft was referenced as early as the 9th Century by Celtic monks and has long been regarded as one of the chief portals to the Otherworld – a dimension to which the living could travel.

Though the cave is currently closed to visitors for health and safety reasons because of its elevated carbon dioxide levels, I get down on my hands and knees to look inside the entrance to the passage, which is framed by an Ogham stone dating back to the 4th-8th Centuries. From here, the 10.5m-long (34ft) narrow stone passageway leads down to the natural cave. An image on the new information panel at the cave's entrance shows what it looks like underground and explains how this "infamous" cavern was "regarded as one of the chief entrances or portals to the Irish Otherworld" – a mythical place "where pre-Christian societies believed the Sun went at night.”

As we continue walking along the trail, we come to the 115m-wide (377ft) circular enclosure of Rathnadarve, where the two bulls meet in mortal combat at the end of The Táin, the Irish national epic. Further on, we cross a narrow country road and see a sign hidden pointing to a grassy mound. It reads "Cruachan Connacht, ancient royal site". This is Rathbeg, where a new information board reveals it was a "Cemetery for Kings". Though it hasn't been excavated, we learn that members of the Iron Age Aos Dána, a high-status warrior caste, could be buried here.

I grew up in Ireland hearing about Queen Medb, and even seeing images of her on old Irish banknotes. Walking through this landscape brings those stories to life, and I can almost hear the hooves of The Táin’s two bulls, the wheels of Medb’s chariot or the eerie screams of the Mórrígan. Unlike a mere monument, Rathcroghan allows visitors to immerse themselves in this landscape and understand how our ancestors used it 5,500 years ago.

"People are used to the fact that monuments of certain types were places of ceremony and ritual," says Wadell. "Rathcroghan offers the notion that whole landscapes could be places of this sort of ritual memory and activity."

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