Mr Brosnahan said the woman pleaded guilty to neglect because she acknowledged she should have known where the child was, but was "too intoxicated to exercise her responsibility" in that area.
Judge Large said the woman "delegated" the responsibility for looking after the baby to another person on December 26, "who returned the child to the defendant's car by putting the child in the back of the vehicle".
Police found the woman "in a drunken state" near the vehicle and took her and another person away, locking the car, not knowing the child was in it.
"It was her responsibility to know where her child was at all times and in that regard she failed," Judge Large said.
"In any event, she was too drunk to know where he was and who was looking after him."
The woman was also sentenced on charges of driving while suspended, supplying methamphetamine, and supplying cannabis, all of which she earlier pleaded guilty to.
Those charges were included in the 18 month prison sentence.