1.  1850 Census.  Napa Valley, Napa Co., CA, p. 133A, 109/109, enumerated 15 Oct 1850 (online in the USGenWeb Archives):

Enoch Cyrus	54	M	Farmer	2500	Tennessee
Rebecca Cyrus	54	F			N. Carolina
Plasant Cyrus	21	M	Farmer	1000	Illinois
John Cyrus	19	M			Illinois
Rachel Cyrus	17	F			Illinois
Jesse Cyrus	13	M			Missouri

W.H. and Mary (Cyrus) Nash are living four doors down.

2.  J.M. Guinn.  1904.  History of the State of California and Biographical Record of Coast Counties, California.  Chapman Publ. Co., Chicago.  Biographical sketch of William LaFayette McCray, son-in-law of Rachel Cyrus. 

3.  Hubert Howe Bancroft.  1884-1890.  California Pioneer Register and Index, 1542-1848.  History Co., San Francisco, CA (online at Ancestry.com).  On p. 113:

Cyrus (Enoch), 1846, overl. immig. with wife and 3 sons, who settled in Napa Valley. v. 529. C. and two of the sons died of small-pox in '53, and the widow, Rebecca Cook, died in '73, age 77, at the farm near Calistoga, where the family had lived since '50. C. (John), 1846, son of Enoch, b. in Ill. '31...

4.  Descendants of John Sires by John A. Mahey. [link died]

5.  At the Donner Party web site, see entry for Lovina Graves. [link died]  The Cyrus family was in a wagon train just days ahead of the ill-fated Donner Party.  The Cyruses made it safely over the summit.  Tradition holds that the Cyrus's eldest son, Pleasant, rode back to the Donner wagon train to urge them to hurry on.

6.  Broderbund.  World Family Trees.  Vol. 2, Pedigree #5730.  Citing Patricia Boland Cartwright, 18 Dec 1961:

On April 16, 1846, the Enoch Cyrus family, members of a train ot three wagons, left St. Joseph, Missouri, bound for Yountville, California, having leased land from Mr. Yount for the purpose of raising grain.       Joining additional wagons along the trail, the wagon train crossed the plains at roughly the same time as the Donner Party.  However, they traveled with considerably more speed, averaging six to nine miles per day, despite the fact that many suffered from Mountain Fever during the crossing.  At the banks of the rivers they removed the wheels and floated the wagons to the other side.  Since some families were bound for Oregon, they crossed the mountains north of Donner Pass, at Chimney Rock, on the Fourth of July.  On Hallow'een, October 31, 1846, Enoch and Rebecca Cyrus and their four small children arrived at their destination where they built and resided in a one-room log cabin.

7.  Broderbund.  World Family Trees.  Vol. 14, Pedigree #179.

8.  For more on the Nashes, see Descendants of John Nash by George E. Nash. [link died]   He gives members of wagon train:

The wagon train with our relatives left St. Joseph, Missouri on April 16, 1846, and arrived in California the last of October 1846. There were three wagons at first. The immigrants included:  Enoch and Rebecca Cyrus and 4 children. (relatives)  Henry Owsley and Frances Cyrus Owsley and 4 children (relatives)  William H. Nash and Mary Cyrus Nash and 2 children (relatives)  E. Kellog and wife and 3 children  Mr. Hillingsworth and wife and 3 children  James Smith and wife and 3 children  Calvin Musgrave and wife and son  Grovenor Boggs and family  Mr. Imman and family  Frank Kellogg  William MacDonald  Mr. Lewis  Manerva and Ann Fowler

Most of these familys show up as neighbors in the 1850 Napa Valley census.

9.  Messages in the Cyrus Family Genealogy Forum (online at GenForum.com).