Mobile support is built into Discourse (the new board software), so mobile access is with your mobile browser. No third-party app needed (and maybe none available?).
On my iPhone (and I presume generally on smaller-screen mobile devices), the site uses a slightly different interface, that drops the "secondary" stuff that appears on the right side of the screen on my Mac - for instance, the Categories page displays the categories list, but not the "Latest" list that's on the right side on the Mac. I can switch to that view, but you get one at a time (makes sense, given the limited screen real estate).
Based on very limited testing with my first-generation iPad (which can barely run a browser...), on larger-screen mobile devices, the site seems to use the same general layout as on PCs/Macs. I haven't got the hardware/software to test it, but depending on the browser you use on an iPad, you may be able to have the browser "pretend" it's on an iPhone, if you want to use the interface that's engineered for smaller screens (with non-Safari browsers, you can control what "user-agent" the browser represents itself as to web sites).