His body was found in a flat on Parnell Street on Saturday afternoon.
At the time, police said they were investigating "all the circumstances" of his death.
The result of a post mortem examination were released by police on Sunday. Police said they are not treating his death as suspicious and will now prepare a report for the coroner.
A two-year-old boy found dead in an apartment in Limerick died from a viral infection, Irish police have said.