The Collaborative International Dictionary of English v.0.48:

Tanist \Tan"ist\, n. [Ir. tanaiste, tanaise, second, the second
   person in rank, the presumptive or apparent heir to a
   prince.]
   In Ireland, a lord or proprietor of a tract of land or of a
   castle, elected by a family, under the system of tanistry.
   [1913 Webster]

         This family [the O'Hanlons] were tanists of a large
         territory within the present county of Armagh. --M. A.
                                                  Lower.
   [1913 Webster]