A fairy fort is a special place in Ireland where people believe that fairies live. It's not a castle or a building like a house, but it's actually a big circle of rocks or trees or sometimes just an invisible boundary.
The fairies are magical creatures that people believe have special powers, and if you go into their fairy fort you might disturb them or make them angry. That's why people are very careful not to disrupt the fairy fort and they don't build anything on it or move the rocks or trees.
When people visit a fairy fort, they might leave a gift for the fairies like a shiny coin or a pretty flower. It's believed that if you show respect to the fairies, they will bring you good luck and protect you from harm.
So remember, if you ever come across a fairy fort in Ireland, be sure to be respectful and leave a little present for the fairies so that they will bless you with their magic.