A rare bright light - which has come to be known as the "Christmas star" - will be visible in the night sky later.
On the winter solstice, Jupiter and Saturn will be closer together in the sky than they have been in 400 years - creating the 'Great Conjunction'.
It's been 800 years, meanwhile, since this phenomenon has happened at night, giving people around the world an opportunity to see it.
From Earth the two planets will appear to be very close together, despite being hundreds of millions of miles apart.
NASA says these 'conjunctions' can happen on any day of the year, so it's a rare coincidence that it is taking place on the winter solstice.
It has been speculated by some that a similar astronomical event could be the explanation behind the Star of Bethlehem, hence the 'Christmas Star' nickname.
Space commentator Leo Enright told Newstalk Breakfast the conjunction comes on a day which is already a 'mystical, magical' day in the sky above us.
He explained: "Unlike any winter solstice in 800 years, Jupiter and Saturn - the two biggest planets in the solar system - are going to be 'kissing cousins'. They're going to be right beside each other in the sky this evening, just at sunset.
"If you haven't great eyesight and you're looking with your regular eyes, you'll probably have trouble even seeing there are two planets in that bright spot in the sky in the south-west."
He encouraged people to look at the sky early in the evening if they want to spot the conjunction, as they will only be in Irish skies in the early evening.
He said: "If you go out at 4pm... if you had binoculars you'd probably be able to pick them out low in the south-western sky.
"At about 5pm, when the sun just about disappears, it should be visible to anyone who isn't clouded out.
"You really want to get into a position where you can see clearly down to the south-west, and look for this beautiful appearance of these two huge planets."