In the speech of older native speakers, I think it’s like this:
Nominative and dative singular : teangaidh
Genitive singular: teangtha (pronounced as if “teanca”)
Nominative and dative plural: teangthacha (pronounced as if “teancacha” : chahng-kah-huh)
Genitive plural: teangthach (pronounced as if “teancach”: chahng-cahh)
It might be different in the speech of younger speakers, quite often, declensions are simplified and may also be influenced by school/media Irish…