We estimate personal expenditure, government expenditure, spending on capital investment and exports. The latter contributes to Ireland's GDP because it is spending by other countries on Irish produced goods and services. On the other hand, we subtract imports from our calculation because this total is spending by Irish persons on the outputs of other countries. These two estimates should give exactly the same answer but, because of the different sources used, and the degree of estimation involved, never do. This is the experience in all countries.
Countries resolve the problem in various ways.
Ireland accepts the income measure as the better measure and adjusts the expenditure estimate to agree. In making these estimates, CSO uses a wide variety of sources. Almost all of the different inquiries conducted by CSO are used in one way or another as well, of course, as providing information about their own subject matter. For example, employment totals from the Quarterly National Household Survey help in calculating total wages and salaries, the Household Budget Survey is used in estimating personal spending and so on. The single most often quoted figure from the national accounts is probably the growth rate.