The Collaborative International Dictionary of English v.0.48:

Stirrer \Stir"rer\, n.
   One who, or that which, stirs something; also, one who moves
   about, especially after sleep; as, an early stirrer. --Shak.
   [1913 Webster]

   Stirrer up, an instigator or inciter. --Atterbury.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stirrer
    n 1: a person who spreads frightening rumors and stirs up
         trouble [syn: scaremonger, stirrer]
    2: an implement used for stirring