For singular masculine nouns, that’s correct for the nominative case. For singular feminine nominative and singular masculine genitive nouns, the noun needs to begin with S followed by either a vowel, L, N, or R — one wouldn’t use e.g. an tscoil, despite scoil being feminine. Note that the “plastered T” even applies to nouns in prepositional phrases, e.g. ar an tsráid, sa tsúil, while many prepositions prevent the t- being prefixed, e.g. ar an oileán, san uisce.