Mrs. O'Leary's Cow can refer to
The cow belonging to Mrs. Catherine O'Leary that folklore blamed for starting the Great Chicago Fire of 1871
http://en.wikipedia.org/wiki/Mrs_O%27Leary%27s_Cow
Catherine O'Leary (also known as Cate O'Leary) (b. circa 1827, d. 1895) was an Irish immigrant living in Chicago, Illinois in the 1870s. On the evening of October 8, 1871, a fire started in her barn at 137 DeKoven Street which went on to burn a large percentage of the city, an event known as the Great Chicago Fire. She was married to Patrick O'Leary. The couple's son, James O'Leary would grow up to run a Chicago gambling hall.
Although for many years it was believed that the fire began with the O'Learys' cow knocking over a lantern, more recent theories posit that Daniel "Pegleg" Sullivan or Louis M. Cohn, who may have been gambling in the barn with the O'Leary's son, James, were involved in the start of the fire.
However, the story of Mrs. O'Leary's Cow has garnered the attention and imagination of generations as the cause of the fire. Popular culture, such as Gary Larson's cartoon The Far Side or Brian Wilson's song "Mrs. O'Leary's Cow," have referred to the story with the expectation that the populace will understand the reference.
Within days of the fire, people were singing the now-familiar, but anonymous song:
Late one night, when we were all in bed,
Old Mother Leary left a lantern in the shed;
And when the cow kicked it over, she winked her eye and said,
"There’ll be a hot time in the old town, tonight."
http://en.wikipedia.org/wiki/Catherine_O%27Leary
full lyrics:
http://www.traditionalmusic.co.uk/child ... _Leary.htm
Still, Egg doesn't get what Barks meant with "overshoes in Mrs. O'Leary's hash".
