different exit-entry China

Hi,
my name is Rob from Dublin.

I have a trip to Beijing. The tiket will be Dublin- Hong Kong - Beijing - Dublin.
I will have already visa to enter in PRC and plan to go to Hong Kong for a week. From HK to Shanghai by train and then , still by train, to Beijing. From Beijing fly to Dublin.
It is that possible or if enter to PRC via Hong Kong i can't leave from Beijing but need to go back to Hong Kong ? In this case i should have Dublin - Hong Kong return
Thank you
Rob