The morning after Lockwood sleeps at Wuthering Heights, he hears Hareton Earnshaw stumble into the kitchen, swearing under his breath:
A more elastic footstep entered next, and now I opened my mouth for a ‘good morning,’ but closed it again, the salutation unachieved; for Hareton Earnshaw was performing his orisons, sotto voce, in a series of curses directed against every object he touched[.]
"Orisons" is a somewhat archaic term for "prayers," so when Lockwood refers to Hareton's ill-natured grumbling as "orisons," he's using verbal irony—Hareton is cursing, essentially doing the opposite of praying.
Lockwood's narration often uses mildly ironic and elevated diction, like "performing his orisons sotto voce," to indicate Lockwood's gentlemanly status and his more detached perspective. In addition, though, there's a marked tone shift between the previous night and this morning. Following Lockwood's nightmare-ridden stay at Wuthering Heights, he's no longer giving its residents the benefit of the doubt; he's feeling rattled by what he's experienced there and, accordingly, he starts using a more suspicious and sarcastic tone. By ironically attributing respectable, civilized piety to Hareton, he means to imply that Hareton is actually uncultured, rough, and impious, prompting readers to wonder what has made Hareton so apparently brutish.
After Catherine dies, Isabella tells Nelly how Heathcliff has been behaving in his mad grief, ironically likening Heathcliff to a pious Christian:
There he has continued, praying like a methodist; only the deity he implored is senseless dust and ashes; and God, when addressed, was curiously confounded with his own black father! After concluding these precious orisons—and they lasted generally till he grew hoarse, and his voice was strangled in his throat—he would be off again; always straight down to the Grange!
Isabella recently married Heathcliff and has suffered from his abuse, telling Nelly earlier that she doesn't regard him as human. Here, she ironically twists the simile "praying like a methodist" around by alluding to Satan and suggesting that, far from being a prayerful, sympathetic mourner, Heathcliff is a monster and damned.
The Protestant religious movement known as Methodism had only become widespread in England by the mid-1700s; at this point in the story's timeline (the 1780s), "Methodist" was a broad-brush term that could be used to apply to anyone who was fervent in their religious practice, especially in enthusiastic prayer. But Isabella doesn't mean the term at all literally, as her next comments show.
The "deity" of "senseless dust and ashes" refers to the late Catherine. In other words, Heathcliff isn't even praying fervently to God, but to the spirit of his dead beloved. Further, "his own black father" refers to the devil, with whom Heathcliff, in his grief and rage, blasphemously conflates God (and Isabella calls Heathcliff the devil's offspring). The fact that Heathcliff's desperate "prayers" are a mashup of pleading and curses (none of which are directed to God) demonstrates that he is utterly irreligious. Like "methodist," then, "precious orisons" (or prayers) is verbal irony, sarcastically characterizing Heathcliff's blasphemous ravings as sincere piety.