When you leave the stadium campus from the Cusack Stand side, head out the gate, then turn right to follow Foster Terrace to it's junction with Ballybough Road. Right turn to Ballybough Road and it's straight all the way to Parnell Street, via Summerhill, and the top of O'Connell Street.
One of the exits from the Davin Stand drops you on the "O'Connell Street side" of the canal. Apart from that you have to cross it from Ballybough, or Jones Road.