The Irish Famine of 1847-48 resulted in a population decrease of millions. Before the Famine, there were some 10m people living here, afterwards only 6m.
The Irish Holocaust site has more.
Of the 100,000 who fled to Canada in 1847, only 60,000 were still alive one month after landing. Among the 40,000 dead was Henry Ford’s father’s mother who died en route from Cork or in quarantine on Quebec’s Grosse Ile.