The New Grange mound was discovered a few hundred years ago but it wasn't until in the 1960s that serious research was done. They found the entrance to the "temple" quite by accident. The front entrance has a door with a window above. The entrance is lined up exactly with the East. The tunnel goes back from the entrance maybe 100 feet or so and there is a wee room. All year the room is in total darkness. On December 21 - the Winter solstice - the light from the sun will run down the hallway and a sliver of light will reach the wee room for 17 minutes, then complete darkness again.
The upper picture shows the sun coming down the hallway. The lower picture is the sliver of light that reaches the back room on December 21st. There is another entrance that faces directly west and the similar thing happens on June 21st.
There are two more mounds - Knowth and Dowth - that are lined up to have a similar thing happen on the Spring and Summer equinox. It was quite a fascinating place to visit.
No one knows for sure why they were build or for what purpose. One thing that was pointed out was that they were built with some sort of plan. The distance from the floor to the ceiling in all three of these mounds are within inches of each other.