I don't know of any way to specify the IP of a network printer. However the printer may be advertising itself by other means. (I assume your router provides WiFi connectivity.)
Some iPad printing Apps like Print n Share may be able to see the printer and successfully print to it. Contact EuroSmartz for advice on whether their Apps may work in your situation.
I have an HP 2605DN connected by Ethernet to my WiFi router. The HP iPrint Photo App, as well as PrintCentral and Print n Share, are able to see it and print to it.