
1) Celtic lord 2) Irish heir-apparent 
Found on 
https://www.crosswordclues.com/clue/tanist

• (n.) In Ireland, a lord or proprietor of a tract of land or of a castle, elected by a family, under the system of tanistry.
Found on 
http://thinkexist.com/dictionary/meaning/tanist/
 Tan'ist  noun
Tan'ist  noun [ 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. « This family [ the O'Hanlons] were 
 tani...Found on 
http://www.encyclo.co.uk/webster/T/9
  No exact match found.