Chris Coleman's side had been on course to challenge the likes of England, Germany, Spain, Italy and France for the inaugural title in European football's third major worldwide competition but lost out by less than 600 coefficient points.
League A consists of the 12 top-ranked teams: Germany, Portugal, Belgium, Spain, France, England, Switzerland, Italy, Poland, Iceland, Croatia and the Netherlands.
Countries will be split into four groups of three, with each team facing each other at home and away.
In a three-team group, Martin O'Neill's side could easily find them in the top tier of the Nations League at the first time of asking.
Uefa say that the new worldwide league "creates more meaningful and competitive matches for teams and..."
If, as is likely to be the case with Group A, there are less than four teams who have failed to make it to the Euros, the highest placed Group B team who has yet to qualify will take part in the Group A play-off.
The Republic of Ireland are in League B along with Austria, Wales, Russia, Slovakia, Sweden, Ukraine, Bosnia and Herzegovina, Northern Ireland, Denmark, Czech Republic and Turkey. However, if a whole league does not have enough teams for a play-off, which is conceivable in League A, then the places will be filled by the best-ranked teams from the league below.
What is the UEFA Nations League?
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.
A weird new system will see the four best placed teams in each group who have not qualified for Euro 2020 take part in a play-off with one spot at the tournament each allocated to the four groups.
The winners of the four groups in League A will qualify for the Final Four competition.
The competition, which Uefa say will replace "most friendlies" in global football, is due to begin on 6 September 2018, and will separate teams according to how they are ranked.
All 55 European nations will take part in the competition which starts in September 2018 and will provide four qualifying spots for Euro 2020.