FM Go can NOT print, but you can create a Robot that prints "printer jobs" for you.
If you have a look at this module, you can figure out how to use a Robot, but instead of creating a PDF you can print
If you host your solution on the PC and access the solution via FMGo on an iPad/iPhone as a client, you can access the invoice and providing the printer is an AirPrint compatible printer you should be able to print it (assuming you have scripted the necessary steps).
If the printer is not air print compatible, then printing from FMGo requires the use of a Robot type client to handle the actual print job or a 3rd party print server such as lantronix.
Not using FMGo as a client or not sharing your database, your options would be something like microsoft remote desktop client for iOS where you remotely connect to your FM11 PC and print as if you were sitting at the PC.
Many ways to do what you wish without knowing exactly what your setup is.
I print from FM GO on my iPhone all the time. As noted by others, the printer must be airprint capable and set up for wireless printing so that your iOS device can find it in order to print to it.
If your printer is not airprint capable check the app store for apps by the manufacturer of your printer.
My printer is not airprint capable but it is an Epson wireless model. They have a nice app for iOS so I can print. It's an extra step in the process as I share the PDF to my EpsonPrint app but it gets the job done easily.
From the iPad you can trigger a script on the FM Pro running in same local network.
We did that before. Both iPad and a Mac/PC somewhere are on the same FileMaker Server.
So iPad commits records for invoice, order or whatever. Than you trigger the script on the Mac/PC and pass the record ID. The Mac/PC receives the script call, goes to the record, prints and goes back where it was.