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sionnachfoxmasculine noun
the
an sionnach
after a preposition
an t-sionnach
of the
an t-sionnaich
plural
sionnaich
the (plural)
na sionnaich

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.