Ireland

Three neolithic sites with corridor tombs are present in the valley of the Boyne : Newgrange, Knowth and Dowth. It's possible to visit the two most important Newgrange and Knowth with shuttles (one must expect half a day). Do not arrive too late (not after 3 p.m. or 2 p.m.) or you may not be able to join a guided tour for both sites.
These sites, cradles of the Irish Celtic civilisation, are outstanding and as far as I'm concerned are even greater than Stonehenge (in England) !

Newgrange

The tumulus holds three funeral chambers in its center. The "dome" created in a spiral pileup of heavy flat stones is astonishing for this era : around 3200 B.C. As a matter of fact, the whole site predates the pyramids of Egypt. Moreover, the rooms had remained impermeable since their construction !
The heart of the sepulture can be accessed by a 19 m (21 yards) long corridor which entry is positioned in such a way that during the winter solstice (on 21st december : which is also the date from which the days draw out) the rays of sunshine break through the passage to light the chamber !

Tours are held on December 21st but admission is by lottery as there are more than 25,000 applications for only 50 entrances. Here is a link if you wish to try your luck : knowth.com.

Archeologists have rebuilt a part of the wall with white quartz stones in the same way it's supposed to have been built to sustain the tumulus.

This main tomb was surrounded by other littler tumulus.

In front of the entry, there is a carved stone which was discovered in the site. The carvings have a spiral shape but till now there is no certain explanation to understand clearly this pattern (precursor to the triskel ?)...It is by the opening just above the top of the door that the light broke through during the winter solstice.

Knowth

At Knowth, the tumulus are well preserved with a big one in the center and several litte "satellites".
The Anglo Normans had understood the strategic location of the tumulus, always built on the top of hills, and raised their homes on them. There are also carved stones and even a peculiar logs circular alignment that reminds of Stonehenge, except that this one is made of wood...

The only remains of the old monastery are a round tower with a collapsed top, two churches, and three high Celtic crosses. But what crosses they are ! The highest is not under 6.50 m (21 ft) high ! Their carvings represent biblical episodes. These Celtic crosses were made of three embedded parts : the base, the cross and the upper part. The ring cross was also a symbol of Celtic Christianity. Unfortunately, the patterns of one of the crosses are erased but they are very well preserved on the two others.

Below, Tall cross, which can be seen in the gallery with Muiredach's cross.