Kelly's Irish Brigade is a decision for 
Confederate States of America. It requires that CSA is at war with 
USA, but it does not stricly speaking has to be the American Civil War. The requirements are that war exhaustion is over 4. It reduces war exhaustion by 1 and turns all 
farmers of Dixie culture in New Orleans (ID:139) into 
Soldiers.
It can potentially help win the war, but it is very taxing on CSA's long-term development, so it should be used with caution.