How to Count Ireland
In 1920 The Government of Ireland Act was passed creating Northern Ireland and Southern Ireland. It was followed the next year by the Anglo-Irish Treaty.
The observations of GDP and population on MeasuringWorth before 1920 are estimated to include the output and residents of all of Ireland. Data after 1920, only includes Northern Ireland. Thus there is an artificial drop in the measures from 1920 to 1921. This effects the growth rate comparisons that include years before and after this date. It also impacts graphs of the period.
1921 also happens to be a year of a recession, so even without removing the Irish Republic, GDP went down that year. The Bank of England presents the data both ways. Real GDP fell 9.7% in the rest of the UK and the numbers reported here decrease by 13%. For Nominal GDP, the drop is 17.9% and 21.3%, respectively.