Traditionally, on 11 July bonfires (pronounced “bone-fires”) are lit in Belfast and other places in N Ireland. They are said to commemorate the signal fires lit for William of Orange on his passage to and through N Ireland before his engagement with King James II at the Battle of the Boyne.
In recent years, such bonfires have become vast pyramids of wooden pallets and sometimes tyres, and may be adorned with viciously anti-catholic sectarian slogans.
Wikipedia has a short article about the eleventh night, here. The date of the Battle of the Boyne, given there as 12 July 1690, is wrong; see here. The French troops who supported William were protestant Huguenots, for the catholic King Louis XIV supported James politically and militarily.