Rancho Pecho y Islay was an 8,856.8-acre (35.842 km2) Mexican land grant in the Irish Hills, Montaña de Oro State Park and Diablo Canyon Power Plant in present-day San Luis Obispo County, California.
It includes the southern part of Montaña de Oro State Park and the site of the Diablo Canyon Power Plant.
[3]: Appx, 31 With the cession of California to the United States following the Mexican-American War, the 1848 Treaty of Guadalupe Hidalgo provided that the land grants would be honored.
In 1891, Ramona Hilliard sold the southern portion of the Pecho y Islay ranch to Luigi Marre (1841–1902).
The Marre land was leased to Pacific Gas and Electric Company in the 1960s and 1970s for the construction of the Diablo Canyon Power Plant.