The difference between an Institutional Review Board (IRB) and a hospital's Ethics Committee is that

the IRB focuses on patient care only, and the Ethics Committee addresses both patient care and business practices. 

the Ethics Committee reviews ethics complaints, and the IRB focuses on developing policies and procedures. 

the IRB deals with the ethical treatment of human research subjects, and the Ethics Committee covers a wide range of issues. 

the IRB is made up entirely of patient care providers, and the Ethics Committee is multidisciplinary.