Irencrag Feat's effect doesn't allow you to cast any spell you couldn't otherwise cast. For example, if an effect says you can't cast more than one spell each turn, Irencrag Feat won't let you cast a second.

Irencrag Feat's effect doesn't care how many spells you've cast so far in the turn. You can cast only one more.

Irencrag Feat's effect doesn't stop you from activating any abilities or from playing lands.

If an effect allows or instructs you to cast another spell after you cast one more, you can't do so. The restriction of Irencrag Feat takes precedence over that permission.