Place any one of these three codes in the head section of the main page of your site.
<script type="text/javascript">
<!--
if (screen.width <= 800) {
window.location = "http://m.domain.com";
}
//-->
</script>
OR
<script type="text/javascript”>
<!-- if (screen.width <= 699) { document.location = "mobile.html"; } //--> </script>
OR
<script language=javascript>
OR
<script language=javascript>
<!-- if ((navigator.userAgent.match(/iPhone/i)) || (navigator.userAgent.match(/iPod/i))) { location.replace("http://url-to-send-them/iphone.html"); } --> </script>
You may edit and add iPad instead of those above or add the line again