A victory by a single goal (1-0, 2-1, etc) over Saudi Arabia would be enough to put Ireland through (they would finish with the same goal difference as Cameroon but would have scored more goals).
The Republic of Ireland must beat Saudi Arabia to have any chance of reaching the second round.
A victory by a two-goal margin (2-0, 3-1, etc) over the Saudis would be enough for Ireland to advance to the second round, regardless of the result between Germany and Cameroon.
If Ireland beat Saudi Arabia they would go through to the second round, regardless of the margin of their victory, if there is a winner in the Germany v Cameroon game (i.e. if the game doesn't end in a draw).
If Germany and Cameroon draw and Ireland beat Saudi Arabia the three teams would finish level on five points - in such an event Germany would definitely be through (their goal difference would be greater than Cameroon's) but the margin of Ireland's win would determine whether they beat Cameroon to second place.
If Cameroon and Germany draw 1-1 on Tuesday and Ireland beat Saudi Arabia 1-0, Cameroon and Ireland would finish with identical records - lots would then have to be drawn to determine which of them would join Germany in the second round.
Drawing conclusions
Germany 0, Cameroon 0
Germany 1, Cameroon 1
A 1-0 win would leave Ireland with an identical record to Cameroon and because the teams could not be separated by 'goal difference', 'goals scored' or the 'result between the teams' lots would have to be drawn to determine which of them would finish second in the group behind Germany.
Germany 1, Cameroon 1
A win by a two-goal margin would put Ireland through at the expense of Cameroon because of a greater goal difference.
Germany 2, Cameroon 2 or any other score draw
Ireland would need to beat Saudi Arabia by a two-goal margin.
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