The Collaborative International Dictionary of English v.0.48:

blarney \blar"ney\ (bl[aum]r"n[y^]), n. [Blarney, a village and
   castle near Cork in Ireland.]
   Smooth, wheedling talk; flattery. [Colloq.]
   [1913 Webster]

   Blarney stone, a stone in Blarney castle, Ireland (built in
      1446), said to make those who kiss it proficient in the
      use of blarney.

   Note: The origin of the stone is uncertain. In order to kiss
         the Blarney stone, which is located in the side of the
         castle, one must be held upside-down by the feet and
         lowered into the proper position from an opening in an
         overhang in the parapet. It is an experience eschewed
         by some tourists.
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

Blarney Stone
    n 1: a stone in a castle in Ireland that is said to impart skill
         in flattery to anyone who kisses it