Rancho San Marcos
Rancho San Marcos was a 35,573-acre (143.96 km2) Mexican land grant in present day Santa Barbara County, California given in 1846 by Governor Pio Pico to Nicolas A. Den and Richard S. Den. The grant in the Santa Ynez Valley extended between the San Rafael Mountains and the Santa Ynez River.
The former Santa Barbara Mission land was granted to the Den brothers, who emigrated from Ireland. Nicolas Den came to Santa Barbara in the 1836 and was also the grantee of Rancho Dos Pueblos. His younger brother Richard Den came to Santa Barbara in 1843, and was a doctor who practiced in the Pueblo de Los Angeles.
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. As required by the Land Act of 1851, a claim for Rancho San Marcos was filed with the Public Land Commission in 1852, and the grant was patented to Nicolas A. Den and Richard S. Den in 1869.
Nicolas A. Den
Nicolas August Den (1812–1862) was an Irish immigrant who moved to Santa Barbara in 1836. Den who had studied to become a medical doctor in Dublin, and although not yet graduated, he became the first medical doctor in the Santa Barbara area. His brother, Richard, came to Santa Barbara in 1843. While both Den brothers were medical men, Nicolas Den’s practice had been put aside in favor of administration of Rancho Dos Pueblos and service as alcalde of Santa Barbara. He also devoted himself to preserving the local mission and establishing a Catholic seminary in Santa Barbara. In later years Nicolas Den was one of the seven organizers of the Society of California Pioneers. He married Rosa A. Hill, a daughter of Daniel A. Hill. He died in 1862, leaving ten children.
Richard S. Den
Richard Somerset Den (1821–1895), a native of County Kilkenny, Ireland, had followed his brother Nicolas, to Santa Barbara in 1843. Richard Den practiced medicine in Los Angeles with the exception of a brief sojourn in 1848 - 1850 prospecting for gold in Calaveras County, and a decade’s absence (1854 - 1866) to administer Rancho San Marcos, until 1895. Den also aroused a certain awe in his later days as he made his rounds astride a black charger, dressed in black and wearing a black felt hat atop "a clustered mass of wavy hair as white as snow". A participant in the Mexican-American War, Richard S. Den, served as the Chief Physician and Surgeon for the Mexican forces. He treated Californios and the American prisoners, including Benjamin D. Wilson and his party captured at the Battle of Chino in 1846.  
- Ogden Hoffman, 1862, Reports of Land Cases Determined in the United States District Court for the Northern District of California, Numa Hubert, San Francisco
- Diseño del Rancho San Marcos
- Santa Barbara County Rancho Map
- United States. District Court (California : Southern District) Land Case 364 SD
- Report of the Surveyor General 1844 - 1886
- Thomas F. Prendergast, 2001, Forgotten Pioneers:Irish Leaders in Early California, Chapter 7, p.49-60 Don Nicholas Den, University Press of the Pacific, ISBN 978-0-89875-390-5
- Mason, Jesse D., 1961, Reproduction of Thompson and West's, History of Santa Barbara & Ventura Counties California, Howell North Books, Berkeley, California,Pages 45-47
- The Irish In Los Angeles by Gloria Ricci Lothrop
- Thomas F. Prendergast, 2001, Forgotten Pioneers:Irish Leaders in Early California, Chapter 8, p.61-65 First Foreign Physician, University Press of the Pacific, ISBN 978-0-89875-390-5
- An Illustrated History of Southern California, The Lewis Publishing Company, 1890. pp. 794-797
- Robert Moes, 1968, The Brothers Den, Richard Hoffman