In the meaning "to", used with place names that start with a definite article. Place names that do not start with a definite article take the preposition go instead.
Because this term originated as a verbal expression, the noun it governs is in the nominative case, as can be seen by its mutation effects on masculine singular nouns after the definite article: consonants are not mutated (e.g. go dtí an capall "toward the horse") and vowels take t-prothesis (e.g. go dtí an t-uisce "up to the water").