You would need to do device detect on the main html site and redirect people that way. Here is an answer to a question that covers all the bases of device detecting (javascript, php, etc) since you can't rely upon just one working:
http://stackoverflow.com/questions/9511526/auto-mobile-site-redirect
Keep in mind that device detection is looked down upon by a large number of people (just like anything else... it's a double edge sword). Also keep in mind that device detection is NOT RELIABLE by any means. It can easily be tricked (not that many people would do this, but you'd be surprised).
Also, a large majority of people do not like being forced to view a mobile version of a website since their new smartphone is probably more than capable of showing their full site version just fine. It's probably best to give the user the option instead of forcing it upon them.
One other thing to note... it's MUCH better to use media queries and "capability" detection rather than device/browser detection methods. For example, add functionality to your website based on if the device/browser can handle that functionality. This is much more reliable and doesn't "sniff" the user for information.
Read up on Luke W. and other great CSS authors to learn more (no I don't make money off endorsing them, I just respect the movement they are trying to make). This one in particular is great, but check out his site for way more information than you'll ever want to know :)