>

You're assuming they stay in Britain instead of say, migrating to America or elsewhere
What am I missing here?

That historical records and community estimates are based on hand counted censuses whereas your analysis is based on a lot of mathematical assumptions that didn’t actually happen. >

Are you arguing that your estimates are better than census records? >

The British census data only includes Irish-born.
426,000 were irish-born in 1901, fifty years after the Famine.  The 2 million is an estimate.
But that doesn't include an unspecified number of second and third generations.
The historians and demographers come up with 2 million, but two cohorts born over a century (based on number of children born per women) would suggest 3x time as many offspring.