There is much to be said for a country of Ireland's size having only one embassy in Rome. The one I have in mind played a vital role in World War Two, rescuing thousands of Jews and allied soldiers from Nazi hands.
The major player in the rescue effort was, of course, the estimable Monsignor Hugh O'Flaherty. But he could not have achieved his work without the assistance of the Irish Vatican embassy. The monsignor was subsequently declared "Righteous Among Gentiles" by the Israeli state.
Due to his efforts and those of Pope Pius XII, the historian Martin Gilbert estimates that approximately 85pc of Italy's Jews survived the war, compared to approximately 80pc of European Jews in general who perished. On this basis alone, ignoring the fact that the Vatican is still the destination of choice for the vast majority of Irish people visiting Rome, it is apparent the Irish embassy to the Holy See is of more practical use than its "secular" counterpart.