The event changes European Championship qualifying, with one team from each Nations League division to reach Euro 2020, with the other 20 places going to the top two sides from the 10 groups of the subsequent, traditional qualifiers.
The teams will be divided into four groups of three, with the four group winners, or Final Four, then contesting the Uefa Nations League Finals - semi-finals, third-place match and final - in June 2019.
UEFA first approved the tournament in 2014, and has chose to split the 55 countries into four so-called "leagues", based on its coefficient rankings.
The competition, which Uefa say will replace "most friendlies" in worldwide football, is due to begin on 6 September 2018, and will separate teams according to how they are ranked.
But it also means there is likely to be some more convoluted permutations as the Euro 2020 qualifiers draw to a close about who or who could not make the play-offs.
The results of the past week's World Cup qualifiers cemented each country's starting position in the Nations League - a biennial four-division tournament which replaces friendly matches from September.
The overall Nations League rankings will also be used to decide the seedings for Euro 2020 qualifying.
The 12 best countries will compete in League A and will be split into four groups of three.More news: Tom Brady Returns To Patriots Practice
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Ireland will be joined by Wales, Austria, Slovakia, Russia, Sweden, Ukraine, Bosnia, Northern Ireland, Demark, Turkey and the Czech Republic.
League A will consist of the 12 top-ranked teams.
League C: Hungary, Romania, Scotland, Slovenia, Greece, Serbia, Albania, Norway, Montenegro, Israel, Bulgaria, Finland, Cyprus, Estonia and Lithuania.
The group winners in Leagues B, C and D will gain promotion and those who finish bottom of the groups in Leagues A, B and C will be relegated.
The national teams of Ukraine and Russian Federation will be divided into different groups.
The winners of the four groups in League A will qualify for the Final Four competition. The four group winners will be promoted to League C for the 2020 edition.
The finer details of the Nations League can be found over on UEFA's website.
The qualification for Euro 2020 will now now run from March to November 2019, with Dublin hosting the Group draw on 2 December 2018.