I'll have a go, not sure I fully agree with CreditLimit's explanation.
GDP is the goods and services produced in Ireland.
GNP is the GDP less income paid abroad plus income received from abroad.
Thus GNP is adjusted downwards for payments of interest on debt and repatriations of profits etc. to foreign investors and adjusted upwards for any such interest/dividends etc. flowing into Ireland from abroad.
GDP is a measure of domestic economic production.
GNP is a measure of domestic income generation.
They should be broadly equal but in Ireland's case the multi nationals take a significant chunk of our GDP in profit repatriations. There are strong suspicions though that these profits are artificially bloated by transfer pricing and that Ireland isn't really fully producing the goods and services behind these profits. It is only because of this possible distortion that one can really argue that GNP is a better measure than GDP, otherwise the answer depends on the context.
For example which is the best indicator of the Government's tax base? Normally that should be GDP. However, the artificiality of our GDP would render this answer misleading for policymakers, for if the Government sought to tax those multinational profits at say 20% they would quickly find the multinationals and the transfer pricing distortion disappear.