Magic and Ceremony
Getting to the bottom of things
In pre-Roman Britain, both wells and pits held significant
religious meanings.  They were closer to the underworld and
thus allowed for communication.
Don't get stuck on ceremony
A Celtic site in Milton revealed an apparently ceremonial stick
with a flat, bronze arrow head-like top with oval ends on the
bottom.  Each oval end had a ring, possibly to hang bells from it.
Sacrifice
There is definitive evidence of human sacrifice; both as an offering
and to use entrails for divination purposes: The Lindow Man,
found in a bog, had mistletoe in his stomach.  There were bodies
found in Danebury which had been weighted down by stones.  
A young man buried under the rampart of South Cadbury was
crammed in and face down; this implies a foundation offering.  
Sheela-na-gig - is a female Celtic goddess who appears at many
sites.  Her symbolism represents both creation and destruction.
Probable Sites of Ancient Ceremony:

Cerne Abbas Giant - is carved into the ground in a 1ft x 1 ft
trench.  He is 180 ft long by 167 ft wide, is holding a long club
and sports a detailed phallus and testes.  The idea of this as a
center for fertility rites is strengthened by the fact Maypole
dances were held in an area just north of the giant's arm.  No
one knows exactly when the image was first cut into the ground.
(England)

Isbister - is a Neolithic Site in Orkney 10 white-tail sea eagles
were entombed with the 340 skeletons. (Scotland)

The shaft well at Ashill in Norfolk - it is made of wood, is
under 4 sq ft but 40 ft deep.  The top half of the well contained
Samhain ware (Roman pottery), bones, wicker and wooden
baskets, iron ware, etc.  Below that were urns put there in
layers surrounded in hazel leaves and nuts.  Early Irish folklore
states a relationship between a religious significance to hazel
(Coll in Old Irish) and it's association with sacred wells.

Emain Macha - also known as Navan Fort although it was
probably a circular temple.  It is associated with King
Conchobar and most likely connected with other sites in the
area.  It seemed to have been used for tribal gatherings and
festivities as well as religious activities.   The whole structure
was set on fire possibly to call the gods during a time of war.
Herbs and their Powers
Mistletoe
The name comes from the Old English for "Special Twig".  According to Pliny
the Elder, the Druids used this poisonous plant and its deadly berries:
it was harvested from oak trees with golden sickles on the 6th day of the
(new) moon, after covering the ground with a white sheet to prevent the
trimmings from touching the ground.  This would make them lose their
power
a sacrifice had to be made in exchange for the power of the harvested
plant
it was used in human sacrifices
the evergreen part of the plant symbolized fertility since it lived through
winter (and the fertility aspect is what is probably reflected in the kissing
under the mistletoe custom)
it was hung over doorways to protect against evil
Hazel
Hazel leaves and nuts are venerated in Celtic society.  They are associated
with wisdom, and when thrown down a sacred well allow sacred salmon to
eat them.  This is why the salmon represents wisdom as well.
Chalice Well
Water, Water Everywhere...

Rivers are associated with fertility to the ancient Celts.  A perfect
example come from the mating of the Morrigan and the Dagda, during
which Morrigan had a foot of either side of the river bank
Not coincidentally, rivers are also associated with Mother-Goddesses as
can be seen from names of multiple British rivers the Braint at Anglesey
and the Brent at Middlesex, both named for Brigantia/Brigit;  the Clyde
for Clota and the Severn for Sabrina.  Ireland also has examples including
the Boyne for Boand and the Shannon for Sinann
There are also wells throughout England, Scotland, Wales and Ireland
which are names for local goddesses/fairies
According to early Irish legends, the feast of Samhain (Nov 1) is to be
celebrated by lake shore
The numerous find in bogs and pits probably ties into this water worship
as sacrificial offerings
Water, pits, wells seem to be entrances to the otherworld, a belief not
confined to this region.  In fact, it's even in the Old Testament.
Water is strongly connected to the gift of "sight", or an ability to see and
understand things from the other world.
The river Boyne was to have magical.  In fact the river's source was
thought to be an well of the Otherworld, Segais. This makes the river
the origin of all wisdom and occult knowledge.  In fact the well was said
to be surrounded by Hazel trees whose nuts fell into the well.  Some say
annually, others every seven years
Chalice Well
Sun Worship
The Sun was also worshipped in both Neolithic and Celtic Society.  
How can we tell?
The Dagda may well have been a sun god - although some
scholars do believe he has a sky god.  He was the most
prominent of the Irish gods.
St. Patrick wrote in his Confessio "The splendor of the material
sun , which rises every day at the bidding of God, will pass
away, and those who worship it will go into dire punishment"
Newgrange and Stonehenge are aligned for the Winter Solstice
sunrise
Dowth, which is located near Newgrange,  is aligned for the
sunset on the Winter Solstice
The Hall of Tara, which in the literature is the seat for the
Ulster kings, is aligned with the Newgrange to mark the
position of the midday sun, according to Martin Brennan in
The Stars and the Stones.
Aonach was an ancient Irish festival marking the sun's full cycle
at 365 days.
Head of the Class
There seems to be a particular
reverence for the human head:
Bran's
was cut off and lived beyond his body,
Cu Chulainn collected them from those
he conquered in battle and placed the
heads on stones,  and hero
Conall
Cernach
bragged of sleeping every
night with a head under his knee.
Irish sources refer to the making of
brainballs, where the brain was
extracted and mixed with lime and
hardened to become cement-like.  
Power of the Celtic Goddesses
In "The Book of the Taking of
Ireland" (Labor Gabala Erenn)
the goddesses Eriu, Banba and
Fotla are triple territorial
goddesses whose names are
attached to the land.  The
goddesses must either be
worshipped or destroyed to
take over the land
A recurrent theme in Welsh
mythology is that the women
names and arms their children.
The goddess Medb (or Maeve)
holds dominance over all men
and is reputed to have an
insatiable sexuality.  
Red Ribbon
The traditional practice of tying knots
of red ribbon onto children's clothing
will protect them from harmful magic
may have its roots in early Celtic
practices.
Walk like a...Celt
In Celtic processions, it is believed that
people always walked clockwise or
deiseal. To walk a procession
counterclockwise or
tuathal would
have been ominous.
The Druid's Glass
Pliny the Elder tells us about this
amulet.  It was a necklace worn by
Druids.  It had an oval, colored ball of
crystal, formed from serpents in
summer.  It was used specifically for
counter-spells.
Symbol of the Cauldron
The cauldron must have been a potent
symbol, because it shows up in
different Celtic myths with different
functions.  To the
Dagda, it produced
an unending supply of food,
Goibniu's
was used for an unending supply of
beer yet
Bran's cauldron allowed the
dead to be resurrected, but without a
soul.