“Everyone is Irish on St. Patrick’s Day”

St. Patrick’s Day is an Irish holiday, but many non-Irish celebrate it, also. Many people in workplaces across the United States try to wear something green on March 17th, “Everyone is Irish on St. Patrick’s Day” is a popular saying.
 
“Everybody is Irish on St. Patrick’s Day, in spirit if not in personal bloodlines” was cited in print in 1953.
 
   
Wikipedia: St. Patrick’s Day
Saint Patrick’s Day, or the Feast of Saint Patrick (Irish: Lá Fhéile Pádraig, “the Day of the Festival of Patrick”), is a cultural and religious celebration held on 17 March, the traditional death date of Saint Patrick (c. AD 385–461), the foremost patron saint of Ireland.

Saint Patrick’s Day was made an official Christian feast day in the early 17th century and is observed by the Catholic Church, the Anglican Communion (especially the Church of Ireland), the Eastern Orthodox Church, and Lutheran Church. The day commemorates Saint Patrick and the arrival of Christianity in Ireland,[3] and celebrates the heritage and culture of the Irish in general. Celebrations generally involve public parades and festivals, céilithe, and the wearing of green attire or shamrocks. Christians also attend church services and the Lenten restrictions on eating and drinking alcohol are lifted for the day, which has encouraged and propagated the holiday’s tradition of alcohol consumption.
 
6 March 1926, The Globe (Toronto, ON), “Listening In: Features of the Coming Week,” pg. 9, col. 1:
Betty Crocker, who broadcasts Mondays, Wednesdays and Fridays from WEAF, believes that every one is Irish on St. Patrick’s Day, and for that reason will describe a typical St. Patrick’s party, including decorations and the food to be served, in her Gold Medal Flour Home Service Talk from WEAF on Monday, March 8, at 10:45 o’clock.
 
17 March 1953, The News and Courier (Charleston, SC), “Hooray for the Irish” (editorial), pg. 4, col. 2:
Everybody is Irish on St. Patrick’s Day, in spirit if not in personal bloodlines. Some all of us Irishmen will be celebrating today in one way or another.
 
3 March 1954, Oshkosh (WI) Daily Northwestern, pg. 12, col. 5:
Everyone Is Irish On St. Patrick’s Day
CHICAGO (UP)—Commissioner John J. Duffy of the Cook County board suggested that only County employees of Irish descent be given a holiday on St. Patrick’s Day.
 
The board overruled him and gave the entire force of 7,000 employes the day off.
 
“On St. Patrick’s Day,” the board said, “Everyone is Irish.”
 
14 March 1954, Chicago (IL) Sunday Tribune, “St. Patrick Day Tea Set by Auxiliary” by Nancy Fitzpatrick, pt. 7, pg. 3, col. 2:
Whether or not the family tree boasts leprechauns and ancestors from Killarney, ‘tis said that everyone is Irish on St. Patrick’s day.
 
Google Books
Gay Culture in America:
Essays from the Field

By Gilbert H. Herdt
Boston, MA: Beacon Press
1992
Pg. 236:
What they really say is that everybody who is involved in city politics is “Irish” on St. Patrick’s Day.
 
OCLC WorldCat record
Is everyone Irish on St Patrick’s Day? Divergent expectations and experiences of collective self-objectification at a multicultural parade.
Author: S Pehrson Affiliation: Queen’s University Belfast, UK.; C Stevenson; OT Muldoon; S Reicher
Edition/Format: Article Article : English
Publication: The British journal of social psychology / the British Psychological Society, 2014 Jun; 53(2): 249-64
Database: From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
Summary:
We examine experiences of collective self-objectification (CSO) (or its failure) among participants in a ‘multicultural’ St Patrick’s Day parade. A two-stage interview study was carried out in which 10 parade participants (five each from ethnic majority and minority groups) were interviewed before and after the event. In pre-event interviews, all participants understood the parade as an opportunity to enact social identities, but differed in the category definitions and relations they saw as relevant.