Birth rate
As of 2017, Niger has the highest birth rate in the world with 49.443 per thousand people. 
Japan has one of the lowest birth rates in the world with 8 per thousand people.
While in Japan there are 126 million people and in Niger 21 million, both countries had around 1 million babies born in 2016.
Irish Free State
According to one report, in 1924, shortly after the Irish Free State's establishment, the new dominion had the "lowest birth-rate in the world". The report noted that amongst countries for which statistics were available (Ceylon, Chile, Japan, Spain, South Africa, Netherlands, Canada, Germany, Australia, United States, Britain, New Zealand, Finland and the Irish Free State). Ceylon had the highest birth rate at 40.8 per 1,000 while the Irish Free State had a birth rate of just 18.6 per 1,000.