After the southern counties of Ireland seceded from the United Kingdom in the 1920s and formed an independent republic, the issue of whether to join the Republic of Ireland or remain part of the UK became the primary source of political division within Northern Ireland.
1920 年代南愛爾蘭脫離英國，成立獨立共和國後，加入愛爾蘭共和國抑或留在英國，就成了北愛爾蘭境內最主要的政治分歧來源。