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seinneadairsingermasculine noun
the
an seinneadair
after a preposition
an t-seinneadair
of the
an t-seinneadair
plural
seinneadairean
the (plural)
na seinneadairean

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.