Complex sentence: There is a separate National Archives of Scotland (formerly the Scottish Record Office), which holds government and private documents relating to Scotlandand a similar institution in Northern Ireland is the Public Record Office of Northern Ireland (PRONI).
Split into simpler sentences: There is a separate National Archives of Scotland (formerly the Scottish Record Office), which holds government and private documents relating to Scotland. A similar institution in Northern Ireland is the Public Record Office of Northern Ireland (PRONI).