In Classical Modern Irish a noun is in the accusative when it follows certain accusative-governing prepositions, or when it is the direct object of a verb.
Prepositions that govern the accusative: amhail, gan, go h-, ‘to’, idir, lé, mar, ré, seach, tar, tré, um.
The prepositions ar, i n-, and fá govern the accusative after verbs of motion, but otherwise govern the dative.
In Modern Irish, the accusative form is rarely used. In most instances, nouns in the accusative position take the nominative form. That is, while such nouns are syntactically in the accusative, they are spelled like the nominative.
For more on how the nominative behaves, see the sections on initial mutation and inflection:
Also see section on The Dative Case.