well you can use event and observer for that
controller_action_layout_load_before
and with in that event you can get visitor country from their IP address
for that you can follow this post
http://stackoverflow.com/questions/12553160/getting-visitors-country-from-their-iphttps://stackoverflow.com/questions/12553160/getting-visitors-country-from-their-ip
and redirect user to appropriate store on their country basis
Hope this will help you