Okay kiddo, an sluagh sidhe so i neamhuin is a phrase from Irish folklore. It means "the fairy host of the unforgiven".
Now, fairies are creatures that people used to believe in a long, long time ago. They were believed to be magical beings that lived in a different world than humans. Sometimes, people thought that fairies could be helpful, but other times they thought that fairies could be mischievous or even mean.
The sluagh sidhe were a type of fairy that people in Ireland used to believe in. They were said to be a group of fairies that would fly through the sky at night. They would come to take people away to their fairy world, and sometimes they were said to be unforgiven spirits who had done bad things in their human lives.
So an sluagh sidhe so i neamhuin means the fairy host of the unforgiven, or the group of unforgiven fairies who would come to take people away to their world. But don't worry, kiddo, fairies aren't real and you don't need to be afraid of them. It's just a story that people used to believe a long time ago.