So ion-icon is an Ionic component. You pass the name of the icon you want to use in the name property, and then it adds the class for that icon based on the mode. For example, if you pass heart:
<ion-icon name="heart"></ion-icon>
on iOS it would be:
<ion-icon class="ion-ios-heart"></ion-icon>
and for material design mode it would be:
<ion-icon class="ion-md-heart"></ion-icon>
So when you pass fa-* to the name property it will perform all of our logic. You should be able to use the i element for font awesome, like:
<i class="fa fa-heart"></i>