As things stand, here's a table showing the current rankings of every team that could finish second in their groups (group, played, country, points, goal difference):

1 B 6 Switzerland 18 11
2 E 7 Poland 18 9
3 A 7 France 16 10
4 I 8 Iceland 16 6
5 B 6 Portugal 15 18
6 G 7 Italy 14 3
7 E 7 Denmark 13 7
8 C 7 Northern Ireland 13 5
9 E 7 Montenegro 13 2
10 F 8 Slovakia 12 5
11 D 7 Serbia 12 3
12 D 7 Wales 11 2
13 I 7 Croatia 11 2
14 I 7 Ukraine 11 1
15 F 7 Scotland 11 -1
16 A 6 Sweden 10 3
17 H 7 Greece 10 3
18 A 7 Netherlands 10 2
19 D 7 Republic of Ireland 10 1
20 A 6 Bulgaria 9 -4
21 F 7 Slovenia 8 2
22 H 6 Bosnia 8 2
23 H 6 Cyprus 4 -7


This is taken from results against the current bottom team in each group. In groups A and B, the team that will finish bottom is unclear, but it makes little difference, except if Bulgaria were to finish 2nd in group A.

Providing Wales beat Ireland, they cannot be caught by Greece, Slovenia, Netherlands or Cyprus - of these Greece are strong candidates to finish second, Netherlands have a chance but have to beat Sweden in their last game. Slovenia are unlikely to finish second as Slovakia host Malta while Cyprus need lots of results to go their way. Also, if Luxembourg finish bottom of group A, Hungary couldn't catch Wales. Scotland would be unlikely to catch Wales barring a victory by 3 more goals than Wales manage.

If Wales only draw with Ireland, we could only guarantee finishing above Slovenia and Cyprus - neither are likely to finish second. If Greece and Bosnia both slip up tonight, then a point would be enough against Ireland. A point would also suffice if Croatia and Ukraine draw (by a similar scoreline or a lower scoring draw). Not sure what would happen if we draw with Ireland and Croatia and Ukraine draw and score 1 more goal each (so 0-0 with us, 1-1 there) as that would mean we'd be level on points, goal difference and goals scored with Croatia. Otherwise, a draw with Ireland means elimination.