The late Duke of Gloucester, who died in 1974 was the last Knight of the Order. He was appointed to it in 1934. He wore the insignia of the Order on visits to Northern Ireland - his secondary title was Earl of Ulster
Today The Queen, as Sovereign, is the only member of the Order - the last non-royal Knight died in 1961 - but so far as I know Her Majesty has never worn the insignia of the Order on her visits to Northern Ireland.
My previous posts about the Order can be viewed at The Order of St Patrick (2011), Banners of the Knights of St Patrick (2012), The Order of St Patrick (2013), Insignia of the Order of St Patrick (2014), The Most Illustrious Order of St Patrick (2015), Order of St Patrick (2016)