What is the name of this children's/teens book I read in 1987-88?

First let me say, this is NOT the book "Grave" by James Heneghan...it
sounds alot like it, but it couldn't have been because I read this
book prior to 1990, probably around 1987-88.  It was hardcover, maybe
9"x7".  I got it from the Rogers City, MI library and did a book
report on it when I was in 8th grade.  It is about a young boy who
went somewhere for summer vacation (maybe grandparents?) and he was
bored.  He went exploring and found some old smokestacks or chimneys
coming out of the ground and he went down in one (I think they were
chimney's...maybe mineshafts?).  He went thru the underground tunnel
and found a big cave with a cottage in the cave.  The boy was confused
about where the smoke went from their cottage chimney.  There were
people living in the cave, they were very pale and thin.  Lack of
sunlight and food.  He became friends with them, they said they were
down there because of the potato famine.  The boy thought that was
weird because there was no famine in his current timeperiod.  I think
when he went down there he kinda went back in time.  He kept visiting
them and for some reason he wanted to bring them topside.  The people
were scared to cross through the tunnel because of the "wolves" in the
dark.  When they do get above ground they start to age very
rapidly...the underground cave somehow preserved them in time.  I
think at the end of the book the boy is very sad because he is trying
to cross a lake with one of the girls from underground and she is
dying on him because she is aging so quickly.  It is possible that the
family was somehow related to the boy.  Please help me figure out what
book this is, I'm starting to think I dreamed it up and though it was
a book.

Hello.

Your book was almost certainly "The Mine of Lost Days" (1974) by Marc
Brandel.

The book has the main elements that you describe: a boy who fell into
chimney, time travel, caves (technically mines).  The setting in
Ireland would explain the connection with the potato famine.

Here's the description from the Library of Congress catalog:
" On a visit to Ireland, Henry falls into a "haunted" copper mine, and
discovers that he and his new friends can travel into the past."
http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=1&DB=local&CMD=010a+74008051&CNT=10+records+per+page

The chimney part is confirmed in this description on Abebooks.com:
"The story of a boy who falls down a mine chimney into the past and if
he changes anything he could change everyone's future."
http://dogbert.abebooks.com/abe/BookDetails?bi=84860534

Buy the book from Amazon:
http://www.amazon.com/exec/obidos/tg/stores/offering/list/-/0397315872/all/
Or half.com:
http://half.ebay.com/cat/buy/prod.cgi?cpid=2960377
Or Alibris:
http://www.alibris.com/search/search.cfm?qwork=4376535&matches=10&qsort=p

search strategy:
LOC catalog: time travel, ireland

I hope this helps.