Yes, I guess, but the links don’t work.
http://tigger.uic.edu/~rjensen/no-irish.htm#FOOT18
This is Jensen’s entry. Even shows the same ad copied in this thread.
Photographs? Just how many photographs? And what does “15”, as cited in the text, prove?
Please see River Hawk’s very rational points.
Go to Google images and search No Irish Need Apply.
One image would be a curiosity. Multiple images, surviving to the present, are evidence that the signs were fairly common (unless somebody can show that, 150 years ago, there were people making a point of searching diligently across the country looking for every single anti-Irish sign, and ensuring that their evidence survived for 150 years).