Opponents of peat harvesting in this country argue that it would alter the ecological balance of our peat-rich wetlands and that, as a direct consequence of this, much of the country’s water supply would be threatened with contamination.
Conclusion
We can safely proceed with peat harvesting (even though some people worry that it would alter our ecology and contaminate our water supply)
Intermediate Conclusion
It wouldn't contaminate our water supply
Evidence (for)
In Ireland, they've peat-harvested for centuries and the water supply is not contaminated.
Evaluate
Since there are technically two conclusions, we could worry about both links:
Does the fact that Ireland has peat-harvested for centuries without water contamination prove that peat-harvesting in this country wouldn't lead to water contamination?
No, because we don't know if Ireland and this country are fair to compare in this regard. Maybe the ecology of Ireland has some advantages that are country lacks when it comes to being able to peat-harvest without contaminating the water.
So we could strengthen the argument by making Ireland and this country more fair to compare.
If we accepted that peat-harvesting wasn't going to contaminate our water, would that prove that it's safe to do peat-harvesting?
No, because water contamination is just one possible threat to ecological safety. Maybe peat harvesting wouldn't contaminate the water but would have some other malign effect.
Goal
Look for an answer making Ireland and this country more fair to compare (what happened there would happen here), an answer ruling out other possible safety concerns of peat-harvesting, or just some other answer that makes us feel like peat-harvesting would be safe.