In the ninth century they built a fort on the Liffey and founded Ireland's first town Dubh Linn or  Black Pool. 
They built a fort on the LIffey that protected Dublin well.
mnli hypothesis: They built a fort on the LIffey that protected Dublin well. premise: In the ninth century they built a fort on the Liffey and founded Ireland's first town Dubh Linn or  Black Pool. 