Bizarre how there are countries (which are not just city-states) where more people lived 180 years ago than there are now. It’s only 1 country, and the difference is still significant: 8,221,292 in 1840 vs 4,986,525 now. The reasons? Well, first of all, the Great Famine in the 1840s. It resulted in 1 million dying and 1 more million fleeing the country. The next half a century was not that desperate but the conditions on the island were still harsh with rising prices, a lack of jobs and poor harvests. So more people left. There are only around 5 million Irish living in Ireland now, while more than 35 million live in the US, more than 14 in the UK, more than 7 in Australia and more than 4,5 in Canada. So although the population of Ireland itself decreased, the number of Irish people only grew. Or at least descendants of Irish people.
Here’s the source and of course, we don’t exactly know how many people lived in many places in 1840 so although this data might be considered reliable it is not 100% correct.