The best way would be to search the census for your Irish relatives and look at the "employer" section. It should tell you what their occupation was and MAYBE the company. I highly doubt you'll find a list of employees. It would be like trying to ask McDonalds for a list of every single employee they've ever employed. Most of the people who lived in Detroit at that time worked for Ford or one of the other car manufacturers.
You might try looking through the Detroit newspapers for mentions of your ancestors. If you can find them, look through the obituaries as they often times mention a major employer and how long they worked there.