The idiomatic Irish speech represented throughout Ulysses includes some alternatives to blasphemously taking the Lord's name in vain. Gifford notes that expressions like Mulligan's "Janey Mack" and Richie Goulding's "by the law" are forms of "what the Irish call 'dodging the curse': substituting euphemisms for the names of God when cursing (20). Linguists sometimes call these anodyne expressions "minced oaths."
Immersed in smoke while making breakfast, Mulligan exclaims, "Janey Mack, I'm choked!" Gifford glosses this as a commonly used minced form of "Jesus Jack," and he cites lines from a children's rhyme:
Janey Mac, me shirt is black,A similar curse-dodging follows a few sentences later: "Where's the sugar? O, jay, there's no milk." "Jay" here substitutes for Jesus, as does the more familiar "Jaysus" whom Mulligan references in Scylla and Charybdis: "Longworth is awfully sick, he said, after what you wrote about that old hake Gregory. O you inquisitional drunken jewjesuit! She gets you a job on the paper and then you go and slate her drivel to Jaysus. Couldn’t you do the Yeats touch?"
What'll I do for Sunday?
Go to bed and cover me head
And not get up till Monday.
"Jesus Jack"—an expression that I have not seen discussed anywhere—itself sounds like it may have resulted from toying with the phonemes of some divine name, but the unaltered form of "Jesus" would make it more offensive to pious ears. Molly uses it in Penelope while also offending racial sensibilities: "the one they called budgers or something like a nigger with a shock of hair on it Jesusjack the child is a black." Of the entire phrase, which sounds much like the first line of the nursery rhyme quoted above, Gifford says, "Vincent Deane reports that this is still a popular Dublin catch phrase; its origin is unknown."
In Proteus Stephen remembers his uncle Richie saying, "Sit down or by the law Harry I'll knock you down." The phrase "by the law" here stands in for "by the Lord." "Old Harry" is a nickname for Satan, the devil.