Norwegian Vikings invaded the territory around Dublin in the 9th century, establishing the Norse Kingdom of Dublin, the earliest and longest-lasting Norse kingdom in the British Isles, excepting the Kingdom of Mann and the Isles. This corresponded to most of present-day County Dublin. The Norse referred to the kingdom as Dyflin, which is derived . The first reference to the Vikings comes from the Annals of Ulster and the first entry for 841 AD reads: "Pagans still on Lough Neagh". It is from this date onward that historians get references to ship fortresses or longphorts being established in Ireland. It may be safe to assume that the Vikings first over-wintered in 840–841 AD. The actual location of the longphort of Dublin is still a hotly debated issue. Norse rulers of Dublin were often co-kings, and occasionally also Kings of Jórvík in what is now Yorkshire.