though megaliths - stone circles, standing stones, dolmens, etc. - have been a passion of mine for a few years.

There is really very little known about these remnants of BCE/pre-celtic peoples. Archeologists rely, as most of us know, on gravegoods and what amounts to refuse heaps, unless some catastrophy caused everyday life to come to a standstill, as it did in Pompeii. And the sites where these megaliths are found rarely reveal much in the way of everyday life, or even a hint to their use (unless, of course, they're cairns and dolmens with intact remains).

In part, because of the age, and what's left of Ogham, there is some assumption that pre-celtic druids(?) had some hand in these "structures."

Two excellent sites I can highly recommend are:

The Megalithic Portal
and
Megalithomania

There is much speculation, of course. Everything from sundials and roadmarkers, to high holy day sacred spaces, to lintels/supports for ordinary structures.

Some other links you may find interesting:
Housing in Ancient Ireland
and
The ModernAntiquarian

Enjoy your search!
Ladee