Ireland is not very big. In fact, you can drive from the east to west coast in less than four hours. But the country’s newest tourism initiative – Ireland’s Ancient East – is aiming to cover almost half of it, from Cork 310km north to County Cavan and from the coast 150km west to the river Shannon. There’s no one route, itinerary or even a finite list of sites.

What it does offer is some of the country’s best-known sites – the Hill of Tara, Kilkenny Castle, Newgrange – alongside less-visited local secrets; an absolutely magical combination of the extraordinary and the unknown. And that’s how I found myself on a journey that felt like a choose-your-own-adventure through the past 5,000 years, starting at the four peaks of Carnbane East, Carnbane West, Patrickstown and Carrigbrack, located just 80km northwest of Dublin in County Meath.

From the summit of Carnbane East, a postcard-perfect panorama of Ireland spread out before me. Beyond the hedgerows and sheep in the patchwork of green fields, I could see the three other hills running east to west in a 4km chain that made up what might be Ireland’s finest – and least-known – Stone Age landscape.

Each of the peaks is topped with cairns, adding up to some 30 in total; originally, there may have been up to 100. Built some 5,000 years ago by the first farmers of Ireland, these megalithic mounds make up one of the largest groupings of cairns in the whole country, with the main concentrations on Carnbane West and Carnbane East, where Cairn T is the centrepiece.

Incredibly, the site is all but unknown to the tour buses that ply better-known spots. Guided tours, a year-round staple at other sites, take place here only in summer. The rest of the year, the process is endearingly simple: head to the little café at Loughcrew Gardens, sign a book, borrow the key for Cairn T, drive (or walk) the 2.5km to the small parking lot at the base of Carnbane East and hike 1km up the hill.

At 35m in diameter, Cairn T is large enough to line up three London buses inside. And though the sheep who ambled over the nearby ancient stones didn’t seem to think so, the cairn was incredibly impressive.

Peeking through the metal gate into the stone mound, I could see what I’d come here for: the cairn’s passage tomb, a stone tunnel that leads you deep inside the mound. Taking a deep breath, I put the key in the lock.

It didn’t fit.

That’s the other thing about travelling through lesser-known parts of Ireland’s ancient east: when you’re in areas not aimed specifically at tourists, at sites where you’re expected to be clever enough to sort it out yourself – you need to make sure to always ask for the right key.

After circling the cairn, befuddled, wondering if there was another entrance, I looped back to where I’d started. Patting my fingers around the gate, I breathed a sigh of relief: there was another lock. The key fit in cleanly.

Hunching to avoid smacking my head on the stone ceiling, I walked slowly through passageway, the same one designed thousands of years earlier so that, on the spring and autumn equinox at dawn, sunlight floods the narrow passage and lights up the inner tomb. In the central chamber, the huge stones were carved with intricate loops and patterns, their specific meanings now lost. So was the meaning of the cairns themselves: academics don’t agree passage tombs were all burial chambers, only that they had extraordinary astrological, and likely spiritual, significance. The mystery, of course, only makes them even more fascinating. As did getting to experience one without so much as a guide.

Instead, to find out more about the country’s most ancient secrets, I went to talk to the people who have guarded them for centuries: the locals.

Over a breakfast of homemade soda bread and local eggs at the nearby Lough Bishop House, a 19th Century farmhouse turned B&B, the owner Christopher Kelly offered to give my travel companion and me a tour of the 100-acre farm. Take a site from this part of Ireland and put it anywhere else, and you could charge admission. But here, ancient sites literally make up the landscape. “See that part of the land up there, near the sheep?” he asked, pointing out a ridge in the land. “That’s an Iron Age enclosure.”

It was also Kelly who told us about another must-see in Ireland’s east: Uisneach. The 150-acre property, privately owned like most of the country’s other ancient sites, is believed to have been the most important spiritual centre of prehistoric Ireland. I thought I’d done my research, but Uisneach hadn’t come up, its importance swallowed by the stature of sites like the Hill of Tara and the tombs of Newgrange (also in Ireland’s east, and also well worth visiting).

“It’s very much a matter of searching it out for yourself in this part of Ireland,” Kelly said. “It’s not set up for tourists as much as the other areas.”

So to Uisneach we went. After getting completely turned around – put “Uisneach” into Google Maps and it takes you not to the ancient site, but to a housing development with the same name – we made it. Marty Mulligan, the site’s sprightly tour guide, was there to meet us.

We were, it turned out, in the very heart of pre-Christian Ireland: this was known as the country’s navel, the axis mundi, the point where the five ancient provinces met. It was also where the Earth was said to meet the Otherworld.

And according to oral tradition – and until the arrival of Christianity in the 5th Century, all of Ireland’s history was passed on through stories – it was the final resting place of the Earth Goddess Ériu, who gave her name to Ireland, and of the Sun God Lugh, whose name was lent to London. It was also a site where thousands of people may have gathered each year.

“This was a sacred place. It was like a mecca: People made the pilgrimage here because of the ancient gods,” Mulligan explained. For Bealtaine, the bonfire-filled Celtic festival that heralded summer’s start, the fire at Uisneach was said to be the first one set, signalling to others to light theirs in a chain across Ireland.

People still gather at Uisneach to celebrate festivals like Bealtaine today. But on this day, we were the only ones here, surrounded only by marks of the past.

As we walked through the lush, jewel-green landscape, we passed one mound after another. There are the remains of more than 40 monuments here, including ring forts, barrows, standing stones and a possible megalithic tomb. It’s not clear what any, or all, were used for; oral tradition aside, the most in-depth excavations done were in the 1920s and the site has been relegated to relative archaeological obscurity today. Still, recent academic work has found that periodic feasting and fires did occur here, with artefacts found as far back as the late Iron Age of the 3rd to 5th Centuries, and that a particularly opulent ring fort may have been a royal residence.

We arrived, finally, at the Catstone. The 30-tonne, 6m-tall limestone boulder, carried here by a glacier thousands of years ago, looked like gods themselves had perched it in place. It was this massive formation that was said to be the navel of Ireland, the resting place of Ériu – and the portal into the Otherworld.

And, at that moment, I certainly felt transported into another world – one that belonged to a different time and a different people. It was as far as possible from the Ireland that many travellers get to see.