The first map showing Ireland as separate from Britain has been discovered in a 600-year-old atlas and is expected to sell for more than $3.7 million at auction this month.

Auctioneers at Christie’s in London have said the 15th century map is the oldest known separate, individual map of the island.

The Irish Times reports that the hand-drawn vellum map, which auctioneers claim is “aptly bordered in bright green,” was found in a maritime atlas made in Venice by renowned Italian navigator and cartographer Grazioso Benincasa in 1468.

The maritime "portolan" atlas, which holds seven double-page navigational maps designed for ships used in trade and exploration, was created 24 years before Columbus discovered America.

At the time, travel to Ireland was “the equivalent of space exploration” says Christie’s maps expert Julian Wilson.

He told the Irish Times that the atlas was most likely not used by a sea captain, but was probably commissioned by a wealthy owner to consult in the comfort of his home.

He said the “highly important and intriguing map” was inscribed “Irlanda que Ibernia dicitur" (Ireland which is called Hibernia).

Fifty-seven places are named on the map, including Porto Rosso (Portrush), Limerich, Chorca, Drogda (Drogheda); and Bre (Bray). The capital is not identified, but a settlement named Donvelim may refer to Dublin or Dún Laoghaire.

Only two rivers, Barrow and the Nore, are shown, and some offshore islands such as the Aran Islands and those in Clew Bay are drawn in red, blue, green, and gold.

The “extraordinarily rare” atlas, which also holds maps of west Africa, the Canary Islands, France, Spain, Portugal and Britain, will be auctioned in London on November 19th and is expected to fetch nearly $4 million.