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tuathanachfarmermasculine noun
the
an tuathanach
after a preposition
an tuathanach
of the
an tuathanaich
plural
tuathanaich
the (plural)
na tuathanaich

Every form but the headword, the genitive and the plural is worked out by the spelling rules in the grammar guide, so what you see here is what the guide teaches. Checked by a fluent speaker.