That is a puzzle.
My guess (highly speculative) would be Mary is the sister-in-law of Pat, or in other words, the widow of Pat's brother. The reason it does not say "Widow Sullivan" is that Pat's wife is still alive, and so the name differentiates against the future possible outcome of him dying, and there being two women on the same plot with the same "widow" label. Or less thoughtfully, that it was just written differently than the McCormick entry, perhaps on a whim. Maybe, one was known as "Mary" and one as "widow."
I am a bit reluctant to accept the brother/sister theory. It is my thought that these double names on plots were usually brothers, or less frequently, cousins or uncles/newphews. It is my belief that a sister living with a a brother would just not have her name written, especially if she was young and later going to be married.
I've certainly seen women of a late age listed in some head of household capacity, either on the census, or in legal matters, so perhaps mother/son is a possibility, but I would lean more to something starting with a brother/brother relationship. In remote, sparsely populated townlands, you sometimes see three related males together on the Tithes, with a widow tossed among them by Griffith's.
