>What the archeological record does make certain, however, is
> later Celts certainly USED Newgrange and other cairns, even though
> didn't build them.
Actually, I think this question is still debated.
> So it seems apparent that they did certainly mark the
> passining of the solstices.
Whether or not they did, we don't have the rituals used in those places,
if indeed they were. The deities associated with Newgrange--aka Brugh na
Boinne--are the Daghda (a tribal protector), B/oann (a goddess of
sovereignty and fertility in the broadest sense), and Oengus mac /Og,
their son, (associated with romantic love and devotion, the sun, and the
coming of spring).
The Carmina Gadelica has some devotions to the sun and the solstices
which would be suitable. There is a one-volume translation published by
Lindisfarne Press and available through major booksellers. The
untranslated version is long out of print. However, the Gaelic for some
of the prayers is available on line. Search for "Carmina Gaedlica."