I haven't been to Newgrange but it is definitely on my list of places to visit. It was constructed around 3,200 BC (over 5000 years ago!) making it one of the oldest structures in the world with a solar alignment. Stonehenge was built in stages, but Newgrange is older than the earliest part of Stonehenge by at least 500 years.
I'd like to visit Newgrange on the winter solstice since that is the day that the sun's light shines down the interior passage. There are two roof-boxes made of quartz that appear to have been removed and replaced to allow the light to shine in. This suggests that the people who lived back then used Newgrange as part of a winter solstice ritual. This also makes sense because Newgrange is a burial chamber, and winter solstice is sometimes associated with death of the old year and rebirth of the new year.