The passage at Newgrange was partially illuminated during the Winter Solstice as hundreds of people gathered at the Co Meath site to celebrate the event.
Sunrise was just before 9am, however it was around 9.15am before the sun's rays appeared on the horizon over the Brú na Bóinne.
However, the light did not reach as far as the chamber itself.
Eight lottery winners, including people from the US and Chile, along with their guests, were present in the chamber for the solstice, describing the experience as "magical" and "once in a lifetime".
The astronomical phenomenon marks the shortest day and longest night of the year.
The Newgrange monument is a Neolithic passage tomb, built on a bend in the River Boyne more than 5,000 years ago and is famous for its solar alignment.
At sunrise, for 17 minutes, direct sunlight can enter the monument through a small opening above the entrance called a "roof box", illuminating the chamber of the tomb.
Crowds gathered from about 6.30am at the monument on the Brú na Bóinne UNESCO World Heritage site.
Access to the chamber was limited to just a few members of the public who entered a lottery draw to win a visit.
The solar alignment of the tomb on Winter Solstice has been described as an astronomical finding of global importance.
It was rediscovered by Professor Michael J O'Kelly in 1967.
Speaking before the event, Rosemary Collier from the Office of Public Works said it attracts much attention each year.
She said: "People gather at the ancient tomb to wait and watch the illumination of the passage tomb, just as people did 5,000 years ago.
"The OPW and the National Monuments Service are delighted to welcome lottery winners in person, to join in this experience that our ancestors have enjoyed for millennia.
"We are delighted also that we can invite people from all over the world to share in this event with us, by watching the live-stream online and on television."
A team of geophysicists who carried out new research at Newgrange claim to have made what could be the most significant find at the ancient site in 50 years.
The team from Ireland and Slovakia said their findings raise the possibility of one or two additional chambers to the rear of the monument.
The micro-gravity survey and geo-radar scan identified two features side by side, inside the monument, which they say bear the hallmarks of a collapsed cavity.
The two features are located on the same line as the front chamber.
The findings were revealed in a new documentary broadcast on TG4 last night.
'Rún na Bóinne', by filmmaker Neal Boyle, followed the work of Slovakian geophysicists Prof Roman Pasteka, Dr Pavel Zahorec, Dr Juraj Papco and Ema Nogova, along with Mayo-based archaeological geophysicist Kevin Barton.
The team spent nine days in 2022 using non-invasive technology to survey the site.
Mr Boyle said that the results from the project were more than they could have hoped to find.
"What this project and team has done is put some new pieces on the board in our attempt to understand this monument and the culture who built it," Mr Boyle said.
"And it's given future research a target to consider at New Newgrange."
'Like a dream, really magical'
Just eight people won the lottery to gain access to the chamber in the passage tomb for Thursday morning's winter solstice. Each was allowed to bring a guest with them.
Among those winners was Juan Fajardo, who is originally from Chile but is now living in Ireland.
Mr Fajardo described the experience of being in the chamber as "like a dream, really magical".
"I felt like kind of a wave of energy, a feeling of me to cry, a feeling of connection," he said.
Gracie Speicher from the US was in Ireland visiting her brother - who lives in Dublin - last spring when she entered the draw.
"I've never done anything like that before, I just feel so lucky to be one of the people who were chosen," she said.
Ms Speicher described a beam of light on the ground after sunrise.
"It was absolutely incredible," she said.
Deirdre O'Shea and her mother Cecilia travelled from Killarney in Co Kerry for the event.
"As the sun rose, the chamber got more illuminated. You could see the roof of the chamber which was really surprising," Deirdre O'Shea said.
Cecilia O'Shea described it as an unusual and special experience.
"Once in a lifetime, going back in time I suppose and thinking of the people who built the place", she said.
Laura Dillon from Cork said there was singing and chanting inside the chamber as the group waited for the light to appear.
"It was fabulous, it was really magical", she said.
