How do two copies of Irencrag Feat interact?
You cannot cast another spell after casting the second Irencrag Feat
While I don't think the Golden Rule quoted by Arcanist Lupus in his answer necessarily applies here, simple logic will get you there:

You cast the first copy, which creates a condition &quot;Condition A&quot; that says you can only cast one more spell this turn
Trying to cast the second copy, you need to check existing conditions, namely &quot;Condition A&quot;. Since you haven't cast another spell this turn, you cast the second copy, which creates a condition &quot;Condition B&quot; that says you can only cast one more spell this turn
Trying to cast another spell, you need to check existing conditions, namely &quot;Condition A&quot; and &quot;Condition B&quot;. You haven't cast another spell since &quot;Condition B&quot; has been established, but you have cast another spell since &quot;Condition A&quot; has. This means that casting another spell would necessarily violate &quot;Condition A&quot;, so you won't get to cast another spell this turn.

Keep in mind that cards like this will usually receive rulings upon or close to their release for clarification.