Will a Visa to visit Hong Kong be sufficient to vist mainland also?

I plan to visit Hong Kong from 16 November 2013 to 21 November 2013.
I will apply for one to travel to Hong Kong and wish to know if the same one would allow me visit mainland also.
I don't have any specific dates to travel into mainland and would probably only do a day trip from Hong Kong but I have not booked any trip and probably will not do so until I reach Hong Kong.
My brother will travel with me.
We are both born in Ireland and have been Irish Citizens since birth.
Our visit will be entirely as Tourists.

We are discussing a trip to China next year to include Beijing and other parts.
Would that we get to visit Hong Kong in November 2013 be valid for some months and would it be enough to allow us re-enter China some time next year?