1.  Timothy Howard.  1907.  History of St. Joseph County.  Lewis Publ. Co. (extract courtesy of Sue Gates Davis).  On p. 1065:

Of Pennsylvania Dutch stock, both his grandfather, John Lydick and his father, Irvin W. Lydick were natives of Pennsylvania.  The father (Irvin) grew up on a farm in Perry county, where the district schools afforded him an education, and on the outbreak of the war of the rebellion, being a young man and as yet unmarried, he enlisted as a soldier in Company D, One Hundred and Eighth Regiment of [part of sentence cut off at bottom of page] ... married Sarah Ellen Stroup, daughter of Samuel Stroup, both being natives of the Keystone state. Irvin W. Lydick and wife now reside in Walnut Grove, German township, where they have been well known and highly esteemed for nearly thirty years.  The family are members of the Evangelical Lutheran church.

Note that the "Pennsylvania Dutch" are "Deutsch" (i.e., they are German, not Dutch).

2.  Census Index: Western Pennsylvania, 1870 (Broderbund CD-285):  "Lydick, Irwin."

3.  1880

4.  1890 U.S. Census:  the 1890 census population schedules were destroyed.

5.  1900 U.S. Census Index/Microfilm (online at Genealogy.com):  German Twp., St. Joseph Co., IN, p. 33B, SN1, SD 13, ED 103, enumerated 4/5 Jun 1900, official enumeration date 1 Jun 1900 (extracted by Diana Gale Matthiesen):

1900:  for an explanation of the column headings, please see What the Numbers in the Federal Census Mean (missing columns contained no data).
1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	19	20	22	23	24	25	26	27	28
13	14	Lydick Irvin	Head	W	M	Feb 1847	53	M	27			PA	PA	PA	Carpenter	0	Y	Y	Y	O	F	F	13
		______ Sarah E	Wife	W	F	Jan 1856	44	M	27	1	1	PA	PA	PA			Y	Y	Y

Sarah is fibbing about her age.

7.  Company B - Recruited Indiana County, PA, 61st Rgmt., Pennsylvania Volunteer Infantry [link died]:

Irwin Lydick, enlisted Feb 25, 1864; wounded Spottsylvania

8.  Broderbund.  World Family Trees.  Vol. I.

9.  Emails from Susan M. (Gates) Davis.