The largest monument at Carrowmore, with a diameter of 34, meters is known as Listoghil a site with a fascinating astronomical alignment. The original monument at Listoghil is the massive level platform or tertre which is 50 meters in diameter and may date to around 4,100 BC. The platform has a fantastic view of the local horizon, and it seems that the platform had a long period of use as a place to make astronomical observations. It is interesting to note that the tertre is on a ridge 59 meters above sea level, and located about 200 meters east of the highest point.
It seems that this massive platform had a ring of stones placed around the circumference. Today only one stone remains, but geophysical surveys indicate that there were more, and early researchers such as George Petrie noted at least six remaining of the original circle.
The chamber and circle at Listoghil were probably constructed some 500 years after the platform and outer circle, and seem to date to around 3,600 BC, the date coming from the skull of a 54 year old man who was buried within the chamber.
For half a millennium some form of ritual activities, which seem to have involved lots of burning, took place here before the present monument was built. The circle and chamber are believed to have been free-standing for a period, possibly 200-300 years, with two or three inner rings surrounding the chamber. After this free-standing phase some form of cairn was added to the monument.
Because the cairn was used for a quarry for many years we have no idea what form it originally took; the cairn was plundered in the 1830's and 1840's and the stones were used to build field-walls. The modern reconstruction is largely conjectural.
The Reconstruction of the Cairn
During the excavations which took place between 1996 and 1998, a decision was taken to recreate the cairn around the chamber of Listoghil. To accomplish this, the 1830's field-walls were dismantled and removed and placed within the circle at Listoghil. It seems that no-one had a clear plan of how this was to be accomplished and the restoration project took a number of years. Eventually a passage 13 meters in length and 2 meters wide was created using gabions—stone filled wire baskets—creating a modern entrance which follows the axis of the chamber. A large open space was left around the chamber, which had to be stabilized with a concrete base, leaving the monument free-standing.
The reconstruction remains controversial. Because no sockets of orthostats or passage stones were discovered, there is no information about the original construction around the entrance to the monument. Three large boulders, which were discovered outside the entrance to the chamber, may be the remains of an entry feature. It is also doubtful if the chamber would have been constructed without some form of supporting cairn.
Samhain Sunrise Event
Around 2006 Mark Keane, a guide at Carrowmore, noticed that the orientation of the chamber was directed to a low point in the Ballygawley mountains, just east of Lough da Gé, the Lake of the Two Geese. Following a series of observations it was discovered that the sun rises in this location on two dates in the year, October 31st and February 10th, about a week before and after the astronomical cross-quarter days of Samhain and Imbolc.
A series of observations conducted by guides Mark Keane, Jean Ryan and Pádraig Meehan, observed that the sun, when it reached a gap in the mountains, which they dubbed 'the Saddle', shone through the new passage and illuminated the chamber of Listoghil. The underneath of the capstone, tilted at 6° above horizontal, captures the beam of sunlight, and turns an intense golden colour. The triangular blocking stone in the doorway of the chamber casts a long, thin spear of shadow 1.3 meters long, which bisects this panel of sunlight. As the sun rises in the sky, the shadow shortens into a triangle until finally it merges into the back-stone. The effect lasts until the sun rises above the angle of the tilted roof-slab, and is cut off by the modern cairn.
Due to the modern passage, the event occurs on four mornings on each side of Halloween, depending, of course, on a fine sunrise. However, there is more to the monument and its alignment. It is important to remember that the chamber had a free-standing phase when it was constructed, when it enjoyed a total view of the horizon. After the alignment on October 31st, succeeding sunrises move south along the horizon.
Within days of Samhain the sun is crossing the magical lake at Lough da Gé. Over the following weeks the sun crosses the peaks of Anaghmore, Sileve Da Eán, Sliabh Dargan, each with an unopened neolithic cairn on its summit. By December 21st, the winter solstice, the sun has reached the neolithic passage grave known as the Cailleach's House. The sun remains rising at this position for some ten days before reversing its journey and travelling north along the horizon, again crossing the cairns on the Ballygawley mountains. By February 10th the sun has arrived again at the Saddle and sunlight floods into the chamber and the process is repeated.