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seanmhairgrandmotherfeminine noun
the
an t-seanmhair
after a preposition
an t-seanmhair
of the
na seanmhar
plural
seanmhairean
the (plural)
na seanmhairean

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. Awaiting a fluent speaker’s review.