You don't need to deal with interrupts for writing a USB device driver, if you would like a good tutorial on it have a look at
http://www.linuxforu.com/2011/10/usb-drivers-in-linux-1/
which is an excellent starting point to learn how to write USB drivers;
If you have specific issues as you go along; post them we'll see how it goes :)
EDIT
you can use platform_get_irq or platform_get_irq_byname to get the irq for the device.
EDIT 2:
I also point you to http://lwn.net/images/pdf/LDD3/ch10.pdf for your viewing pleasure :-) Shows you how to retrieve IRQs assigned to devices.