Captain Thomas D. Dunnell (N.Y. & Ill.)

I am searching for info on Thomas D. Dunnell/Dunnill who resided in Illinois in 1870. He was from N.Y. He was a Captain in the Civil War and born around 1835. I'm trying to find out any info I can about him but don't know where to start. Can anybody help me in finding his record? Thank you very much. Ann Stephenson