GDP (Gross Domestic Product at Market Prices) is usually defined/calculated as the sum of total final demand less total imports.
Total domestic demand comprises purchases (including all taxes that may apply) by: Households, Non-profit institutions, Tourists (or rather expenditure by non-residents), and Government. Gross fixed capital formation, changes in inventories and valuables are also included.
Final demand also includes the value of exports (which from a Northern Irish perspective include exports of goods and services to Great Britain). Imports include goods and services imported from Great Britain and will also include expenditure by Northern Ireland residents outside Northern Ireland. The following shows the calculation of GDP (expenditure approach):