Army of Northeastern Virginia USA. I think his first name was Irvin?
Edit - There appear to be sources supporting both versions of his first name.
When I searched for Irwin McDowell, my search engine came back and asked me if I meant Irvin McDowell, so evidently that is the spelling the search engine prefers. But if I tell it that yes, I really do mean Irwin McDowell, it will give me results under that name. The sources that call him Irvin and the sources that call him Irwin clearly relate to the same Civil War general.
Hoosier