"South Ireland" Is Not a Country

"South Ireland" is not a sovereign state. The island of Ireland comprises two distinct countries:

1. Republic of Ireland (Éire / Ireland) — 26 counties, capital Dublin, EU member, euro currency, population ~5.3M. Often incorrectly called "Southern Ireland" or "South Ireland" (a term from the 1920 Government of Ireland Act that never took effect).
2. Northern Ireland — 6 counties, part of the United Kingdom, capital Belfast, pound sterling, population ~1.9M.

The border is open (Common Travel Area). "Southern Ireland" was a short-lived British administrative construct (1921–22) superseded by the Irish Free State.