Robert Irwin list in Mifflin County, Pennsylvania in 1790 Federal Census with 1 free white male under age of 16, 1 free white male over age 16 and 1 free white female.

http://search.ancestry.com/iexec?htx=view&r=0&dbid=5058&iid=4185998_00329&fn=Robert&ln=Irwin&st=r&ssrc=&pid=323652

1790 United States Federal Census about Robert Irwin

Name: Robert Irwin Home in 1790 (City, County, State): Mifflin, Pennsylvania Free White Persons - Males - Under 16: 1 Free White Persons - Males - 16 and over: 1 Free White Persons - Females: 1 Number of Household Members: 3

View Original Record

 

Source Citation: Year: 1790; Census Place: , Mifflin, Pennsylvania; Series: M637; Roll: 8; Page: 135; Image: 329; Family History Library Film: 0568148.

Source Information:

Ancestry.com. 1790 United States Federal Census [database on-line]. Provo, UT, USA: Ancestry.com Operations, Inc., 2010. Images reproduced by FamilySearch.

Original data: First Census of the United States, 1790 (NARA microfilm publication M637, 12 rolls). Records of the Bureau of the Census, Record Group 29. National Archives, Washington, D.C.

Description:
This database details those persons enumerated in the 1790 United States Federal Census, the First Census of the United States. In addition, the names of those listed on the population schedule are linked to the actual images of the 1790 Federal Census. Enumerators of the 1790 census were asked to include the following categories in the census: name of head of household, number of free white males and females, number of other free persons, number of slaves, town or district and sometimes town or district of residence. Learn more...

In looking at Robert Irwin son of John and Mary Irwin, we notice that he has a son, William Irwin born in 1781.  This could be Robert Irwin who served with James Irwin during the Revolutionary War, under Captain David Boals in 1780 in Cumberland County, Pennsylvania.