Despite their stellar qualifying campaigns both Wales and Northern Ireland are destined to be Pot 4 teams when the Euro 2016 Group stage draw is made. Seeding for the groups is based upon the "UEFA National Team Coefficient" which is a points total made up of points awarded for wins, draws and defeats, in the following competitions:-
Qualifying for Euro 2012
Euro 2012 Finals
Qualifying for World Cup 2014
World Cup 2014 Finals
Qualifying for Euro 2016 (Excluding Play offs)
Under this system, despite their appalling qualifying campaign, should the Dutch manage to sneak into the play offs tonight, and win their playoff, they would be a top seeded side for the Euro 2016 draw, mainly based on their qualifying record for World Cup 2014, and their performances in the finals (where they won 6 of the 7 games they played).
England, alongside hosts France and holders Spain, would be top seeds. As would Germany, Portugal and (assuming the Dutch miss out) Belgium.
Italy, Russia, Switzerland and Austria are guaranteed to be 2nd pot teams.
After that it gets a bit murky as there are sides whose fate is yet to be decided. Should Ukraine and Sweden make it through the playoffs they too would be Pot 2 teams.
The Czech Republic and neighbours Slovakia are guaranteed to be Pot 3 sides.
Finally, Iceland, Albania, Northern Ireland and Wales are guaranteed to be Pot 4 sides.
http://www.footballseeding.com/national-ranking-uefa/