At 1880ft it's hard to imagine a more scenic or challenging building site than the summit of Slieve Gullion.

But every day a team including archaeologists, engineers, stone masons and blacksmiths has been trekking to the top of County Armagh's highest mountain to repair its 5,000 year old passage tomb.

Wednesday might mark the autumn equinox but the structure comes into its own on the winter solstice when light floods the chamber at sunset.

It's a phenomenon that has been witnessed for thousands of years – but not last year.

"Last year we unfortunately had to close it because we had an engineering report that said that the roof that had been installed in the 1960s had become so corroded and dangerous that it was close to failure," says Historic Environment Division archaeologist Stuart Alexander.

"We've been working for the past year, planning so we could get up here to get this new roof put on."

That work, it's expected, will be completed in time for the Winter Solstice in three months time.

When the weather is good on Slieve Gullion you can see Dublin – when it's not you can barely see your feet.

During our visit to the site both scenarios were true within an hour.

And although some of the materials were dropped onto the site by helicopter, most of the building materials have had to be carried up the mountain each day.

That's a hike of about 40 minutes from the mountain's highest carpark.

It all makes for an unusual but rewarding workplace.

One of those working on the project is blacksmith apprentice Lucas Adams.

Last month he was working on the restoration of two 18th Century cannons at Carrickfergus Castle.

Since then he has been creating a new roof for this much older structure.

"It's very satisfying being able to go around the country and do a certain type of work in all of these ancient places," he said.

"We had to replace the roof. All the steel beams had been corroded. So it needed four new steel beams with new plates and a new cage for light to come in."

Whilst this is not quite the same as the original Neolithic structure that featured a corbelled flat stone roof, the work is being done to the satisfaction of the HED, a subsection of the Department for Communities.

As the tomb is located in an Area of Special Scientific Interest (ASSI), ecological permissions also had to be secured from the Department of Agriculture, Environment and Rural Affairs (DAERA).

When completed the tomb will look much like it always has – a huge mound of loose rocks on top of the mountain with a small entry point visible from its south-west side.

Inside the passage tomb, project manager Owen Mort is testing that the new roof is not causing any problems to the rest of the structure.

"A number of cracks had developed (over the years) so what we've been doing throughout the project is monitoring whether any movement has been taking place," he said.

To do this he uses a device called a tell-tale which helps monitor any movements.

After taking each round of measurements he'll compare them to older recordings to ensure cracks haven't opened up further.

In the weeks ahead members of the team will return to ensure that the work has been a success, allowing the structure to reopen in time for this year's solstice - which this year falls on 21 December.

And whilst Slieve Gullion relates to the Solstice, Wednesday marks the Autumn Equinox – the halfway point between mid-summer and mid-winter.

That means there will be nearly equal amounts of daylight and darkness.

This arises as the earth's axis is tilted to an angle that sees the sun shine directly onto the equator.

According to Dr Rok Nežič at Armagh Observatory and Planetarium the equinox occurs at a point when days are getting shorter by around four minutes each day.

"It is always around the 21st of the relevant month – so March and September for an Equinox and June and December for the Solstices (longest and shortest days)," he said. "But it isn't quite on those dates and that is mostly to do with Leap Days so this year it is on 23 September, just after 1am.

"It's the one time of the year when the sun does rise exactly in the east and set in the west. This far north it is actually more south-east for sunrise in the winter and north-east for sunrise in the summer. But for the two equinoxes it's east for sunrise and west for sunset exactly."

In the grounds of the Armagh observatory there is a modern interpretation of a stone circle.

According to Dr Rok, "we call this our stone calendar".

"It shows the directions of sunset for the four notable astronomical times – the two equinoxes and the two solstices.

"Many of the stone circles around the country will focus on the solstices because they are more significant but some will also have equinoxes in their design."