While the infant - referred to as Baby A - had his own tri-pillow, it was likely the infant had been disturbed by his parents in the bed, causing him to slip down, pathologist Dr Fintan Garavan commented.
"When he slipped down the tri-pillow, his face became covered by soft bedding."
Babies of that age were only able to lift their heads for minutes before becoming exhausted. The boy's face would have sunk into the pillow and/or bedding, resulting in asphyxia.
Coroner Bain also said Baby A's mother had previously discussed with her midwife unsafe sleeping arrangements, as the midwife was aware of a history of alcohol and marijuana.
The midwife had sought an assurance that the baby would be sleeping in a cot, Coroner Bain said.
"That did not occur and the baby is dead."
The Coroner's Court viewed "co-sleeping and the killing of innocent babies" as a form of child abuse, he said.
"The court will recommend that the Government considers this aspect of child abuse in its legislative reforms.
Repeat cases of co-sleeping in families must also be addressed, he urged.
"These parents should have to prove they are fit to parent more children."