What's the procedure in obtaining 72 hour visa?

Hi, I am flying from Hong Kong to Shanghai on 4 March 2015 and departing for London on 7 March 2015, with a total of 68 hours on the ground in mainland China. I have an Irish passport. I assume I qualify for the 72 hours free transit. Do I have to do anything before I arrive in Shanghai regarding obtaining a 72 hour free transit, or will it be done in Shanghai airport on arrival? What do I have to do to at airport to enjoy the policy, or will it be issued to me at immigration counter?