There was a very large Breen section in the cemetery. As members of the ill-fated Donner Party, Patrick and Margaret Breen along with their seven children had been stranded in the Sierra Nevada for 111 days without supplies during the extraordinary snows of 1846. It is said that they arrived penniless in San Juan and were given free shelter in the mission. Early in 1848, when word came that gold had been discovered in the Sierra foothills, one of the Breen children, 16-year-old John, set out for the goldfields and returned with about $10,000 in gold dust.

In December 1848 the Breens purchased the Castro adobe and 400 acres of prime agricultural land in the San Juan Valley. Thereafter - until 1933 when it became part of the State Historic Park - the old adobe building was occupied by succeeding generations of the Breen family.