Sunny says that some people also use Ishan /iʃan/ as the name of the language, but this is the pronunciation used by foreigners and people not from the Esan tribe.
Sunny also noted that other dialects of Esan pronounce the word omo 'child' as /imo/ and /emo/. It's probable that the /iʃan/ pronunciation comes from the same dialect which says /imo/ for omo.