Non-Muslims are not allowed to enter.
Wikipedia http://en.wikipedia.org/wiki/Kairouan#Mosque_of_the_Three_Gates
More information at Sacred destinations http://www.sacred-destinations.com/tunisia/kairouan-three-doors-mosque
Address Rue de la Mosquée des Trois Portes Kairouan, Tunisia
Coordinates 35°40'41.386" N 10°6'10.465" E