I orignally posted this in L&R forum. We couldn't find anything so I thought I'd try in here. I know this pretty vague. thanks for your help.

 I read this book about 12 years ago.It is not by the same author that wrote the Clan of the Cave Bear, but it is along the same lines. It started off with one couple and went through several generations. The first couple only mated at "mating" night. Then in it tells of how they went from that to being "married".At one point there is a famine and the last couple survives by eating snails.At the end of the book the clan did something against their beliefs and so all the people went to live underground. I know a bit odd but it was a good book.
Maybe you might know the author? If not, no big deal.If I remember,she wrote several of these kinds of books.

what genere would these book be in? I was really into them for a while. this one stood out because it was set in the brittish isles.  that right!!!! It was like before the celts but leading up to them. at the end of the book in the authors notes she told the tale of small people who lived underground and that it was somehow connected to lepercauns.... odd.... i just know i really did like it. Now that i think of it, it might be set in france. definatly Eastern Europe.

Not sure of the genre but it sounds like it could be HF.  Your comment about the it being somehow connected to Leprechauns makes me think Morgan Llywelyn for some reason.  She wrote a few about Irish times.  Sorry if that doesn't help :(