The Collaborative International Dictionary of English v.0.48:

Mirthful \Mirth"ful\, a.
   1. Full of mirth or merriment; merry; as, mirthful children.
      [1913 Webster]

   2. Indicating or inspiring mirth; as, a mirthful face.
      [1913 Webster]

            Mirthful, comic shows.                --Shak.
      [1913 Webster] -- Mirth"ful*ly, adv. --
      Mirth"ful*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mirthfulness
    n 1: great merriment [syn: hilarity, mirth, mirthfulness,
         glee, gleefulness]