Javascript can detect screen size and redirect users if they are using mobile. I’ve found that checking the screen size and redirecting users based on that can cover 99% of the use cases rather than injecting some complicated browser detection javascript libraries.
The screen width of IPAD is 768px and screen width of a bigger mobile is 500px. So it would be safe to say that you anyone browsing the site using mobile phone would have a resolution lesser than 700px.
A simple script like that will work
if (screen.width <= 700) { window.location = "http://your-mobile-site"; }