Confirmation of historical El Niño events
The best article about the 1789-1793 El Niño is probably Grove (1998) Nature study. There is also a lot more information in Grove's 2007 paper. In the second paper, he explores the global consequences of the El Niño event (some evidence is mostly circumstantial), which he describes as being quite severe: famine ("By November 1792 over 600,000 deaths were being attributed directly to the prolonged droughts"), revolution (French Revolution), economic hardship...
He provides rainfall and temperature records and many historical reports as evidence of the conditions.


  While further archival research is needed to more fully characterise the 1789–93 event, the evidence of a strong global impact already indicates that it was one of the most severe El Niños recorded. 


In El Nino History and Crisis Grove talks of the idea of a Mega Niño in relation to the continuous series of El Niño events that occurred during the period 1788-93. 


  In terms of severity,the Indian droughts of 1788-1793 surpassed anything 
  that  had  occurred  since  the  events  of  1685-88,  and  stand  out  as  having  both an unusually long duration and extraordinarily wide effects,   within and beyond  India. Indian  droughts  that  co-occurred  with  El  Ninos  in  the  interim, in   1701-2,   1707-9,   1737   and   1783,   while   serious,   did   not   approach the ferocity  of  the  years 1685-1687  in  southeast  India and  it  was  not  until  1788— 
  1793  that  a  comparable  series  of  extreme  events  took  place.