Using Repeater with a Mask

There's no Repeater function for masks, but you can set up something like this by creating duplicates of the layer and manually positioning/rotating them.

Are you approaching it this way because you're using an image for your iris blade? Just to put this out there, it's certainly possible to pass transparency through in an export or dynamically-linked element.

In your case, the easiest approach is likely:
1. Animate one piece of the blade, positioned the way you'd like it to be.
2. Create a new Null object and parent your layer to that.
3. Duplicate your layer, un-parent the first copy. 
4. Rotate the null until the second copy is in the proper position. (I advise doing the math to see how many degrees this should be. 360 / (# of copies).
5. Repeat until you've positioned all the copies.

This method leaves your keyframes all in one composition, though if you need to make timing tweaks, you'll need to do on all the layers. There are other approaches that could link these properties together, but those might start becoming a bit much for someone on day 3. 🙂 

Let me know if any of the above is unclear. Getting comfortable with parenting and setting up elements in repeating patterns like this is definitely a common thing you'll be doing as you get further into your AE career.