All over the world, people with an Irish background and their friends are celebrating St Patrick's Day today. This will be especially so in the USA, where around 40 million Americans claim an Irish ancestry.
But who exactly are these people? The answer to this may appear obvious, but in fact there is considerable debate on the exact numbers of Irish Americans, Scotch Irish and others whose ancestors came from Ireland.
Brian Walker is Professor of Irish Studies at the School of Politics at Queen's University, Belfast