Them are two good ones, he said.
James Joyce writes this sentence. is that used in an informal way?
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"Them are two good ones, he said," though found in a literary work, would not be used in formal English.
James Joyce is not a good reference for "standard" English. Joyce often used language in poetic and punning ways, with multiple meanings for words. To give you the flavor, here's a fairly straight-forward section of Finnegan' Wake:
"What clashes here of wills gen wonts, oystrygods gaggin fishy-gods! Brékkek Kékkek Kékkek Kékkek! Kóax Kóax Kóax!"
Notice play of words, such as "wills" and "wonts" or "oystrygods" and "fishy-gods". (Do oysters have fishy-gods? Ah, Ostrogoths and Visigoths?)
And he quotes the croaking chorus from the Frogs of Aristophanes, where he might be referring indirectly to Gilbert and Sullivan.
Read Joyce with dictionary, glossary and footnotes in hand (well, wouldn't them be handnotes?), enjoy his encyclopedic writing, peel the layers like allium, but don't use most of his works to learn formal or colloquial English. You might as well learn English from the Drake, Hoffman, and Livingston ditty, Mairzy Doats.