An app like Tapatalk is a very popular app that provides a mobile "interface" to the forum. It simply uses/requires a plug-in on the site and the app does the rest. I don't see support for the forum software that's used here though. It works with vBulletin, phpBB, Xenforo, etc. The cost isn't much either - last time I checked it was about $200/year.
For a little bit more cost, Tapatalk used to make a branded app for sites. All they do is take their regular Tapatalk app, called it "BITOG Mobile" or something like that, and stick BITOG logos in various places.