Passage Tombs, Astronomy and Ritual:
At these latitudes NE is generally aligned with the Midsummer
sunrise, NW with the Midsummer Sunset, SE with the Midwinter
Sunrise and SW with the Midwinter sunset.
The passage tomb at Newgrange, in Meath, is aligned with the Winter
Solstice Sunrise (take a look at this fantastic picture of it from  the
entrance to the tomb)  (Ireland) Courtesy of www.knowth.com
The Cairn T at Loughcrew is aligned at both the Spring and
Autumnal Equinox.  (Cairn L is aligned with Samhain sunrise and
can be viewed below) The sun comes through the entrance and
illuminates a craved backstone for about 30 minutes.  The carvings
clearly show a sun, but take a look for yourself and let us know
what you see!  People have made lunar observations as well, but the
pattern would be based on a lunar cycle which only repeats every
18.6 years. (also called the Saros Cycle)  By the way, Cairn T is the
tallest and most prominent part of the Loughcrew complex.  There
are 4 additional mounds which are aligned with the summer solstice
sunrise there. (Ireland) Pictures courtesy of www.knowth.com

In the Long Barrows near Stonehenge, bodies were laid facing East.
(England)
Aerial view of the Knowth
passage tomb
Winter Solstice Sunrise from
the entrance at Newgrange
Spring Equinox Sunrise at Cairn T
Stone Circles/Rows, Astronomy and Ritual:

Castlerigg - is one of Britain's oldest stone circles.  It has
38 stones in a slightly oval shape with 10 small stones
forming a rectangle on the south-east side of the ring.  
There is one stone which is much bigger than the rest,
and it is aligned with the November 1st (Samhain)
sunrise. (England)
Here are video clips from Castlerigg:
http://www.youtube.com/watch?v=YnoukE6JJtw

Long Meg and Her Daughters - this circle is the 3rd largest
in England and is a single circle with a single oversized
stone.  What caught my attention is that the large stone
(which would be Long Meg) is aligned on the midwinter
sunset.  This day can be referred to as St. Brigid's Day,
Candlemas or Imbolc and is generally Feb. 2 (yes,
Groundhog's Day...it is a holdover of these ancient
religions that we try to see when spring is coming on that
date) (England)

Stonehenge - I know, you were waiting for this one.  The
fact is there is too much information for one of my quick
synopses, so here is a link dedicated to
Stonehenge.
(England)

Nether Largie - has rows of standing stones running NE
to SW with another grouping in the shape of a cross.  It is
believed that the stones can be used to calculate the
moon's minimum every 18.61 years as well as some
maxima. (Scotland)

Sunhoney - the stones at this site are positioned to
represent the moon's phases and in fact it has a singular,
the decorated recumbent stone over which the moon rise
is visible. (Scotland)

Avebury Henge - it is the largest stone circle known in the
world at 28 acres.  There is an outer ring of 98 Sarsen
stones with 2 intersecting rings within.  There are 2 stone
lined avenues (the surviving West Kennet Avenue and the
Beckhampton Avenue) which lead to the main areas, one
of the avenues connects this site to The Sanctuary on
Overton Hill. (England)

The Sanctuary at Overton Hill  - Although it started as 6
concentric circles of wood posts, it was later a double
circle of standing stones.  Evidence indicates it was used
for feasts and death rituals. (England)

Stanton Drew - this site consists of 3 circles, 2 avenues, a
cove of stones and external stone marker known as an
outlier.  The rings are North East, the Great Ring and
South West and the avenues run from the center and NE
rings eastward to the river Chew.  The stone cove is in
line with the centers of the same circles and the outlier is
in line to the NE of the circles.  Another boulder is in line
with the Great Ring and the SW one.  With the help  of a
magnetometer, we now know that in the Great Circle
were a series of pits that held posts.  There were 9 rings
which were concentric with that circle. (England)

The Hurlers - is a series of 3 stones circles running NNE to
SSW and there is a "spread" of quartz crystals in the center
circle.  There is both a round barrow and a portal dolmen
nearby. (England)

Men Scryfa - is a standing stone in Cornwall which has
replaced an earlier grave marker.  What makes this stone
interesting is an early Christian inscription calling the
stone of the royal raven.  The raven, associated with death
or battle is considered a magical bird and worshipped by
warrior societies.  Look for the story of Riolobran
(England)
Video of Cairn L at Longcrew at
Samhain (Oct 31) Sunrise: