I'm not from Ireland but apparently in 1948 they declared that the name of the state should officially be "Republic of Ireland" and did not amend the constitution.
No, they declared "Republic of Ireland" to be an official _description_, not the official name. The name is specified near the beginning of the constitution as Éire, or in the English language, Ireland [1], in article 4, and would require an amendment to change. And while we do them way more often than the US does, we haven't done one for this topic.
https://en.wikipedia.org/wiki/Names_of_the_Irish_state
http://www.irishstatutebook.ie/eli/1948/act/22/section/2/ena...