It is traditional on St Patrick's Day (17 March) for Irish people to wear crosses in their hats. Sometimes these are made of pins, sometimes of green ribbon.
In eighteenth-century Ireland, this is not a purely Roman Catholic custom, nor is it connected with resistance to English rule. Irish Protestants celebrate St Patrick's Day too.
The custom of wearing crosses on St Patrick's Day is kept up by Irish people in London. In a letter to his friend Esther Johnson, Jonathan Swift describes seeing the Mall 'so full of crosses, that I thought all the world was Irish'.
The Mall is in St James's Park. At this date, it is a place where fashionable people go to stroll up and down and exchange gossip with their friends.