Where teams finish level on points, only their head to head results count in determining their final positions. That would mean the only results that matter in determining the top 3 positions would be:
Spain 1-1 Italy
Croatia 1-1 Italy
Spain ?-? Croatia.
If it finished a draw, 2-2 or higher then Spain and Croatia would go through on goals scored
If it finished 1-1, then their respective results against Ireland would be considered, meaning Spain would go through and either Croatia or Italy
If it finished 0-0, Italy would be through as winners with Spain taking second spot, as they did better against Ireland than Croatia did.
Aha - that makes sense
Now for your follow up act... could you explain the other thing mentioned above - how, if this happened, would Italy beating Ireland 5-0 affect who finishes higher out of Spain & Croatia? I'm so confused... I'm glad it's a Friday.