And why, for whatever reason, do I need to come rolling home after I give the dog a bone?
Paddy whack harkens back to the ugly times of Irish oppression, when it was not only acceptable, but even encouraged to randomly hit ("whack") unsuspecting Irishmen ("Paddies"), who could be easily identified by their red hair, constant drunkenness and continuous scheming with the Pope.
Giving the dog a bone -- refers to acts of charity by having relations with unusually comely and unsightly ladies. After such act, one would need to roll a cigarette with the whacky tobaccy, which was also obtained from the Irishmen (a double meaning for the "paddy whack").