What could I use to fill a pattern of small holes in a old fireplace?

Can you post a picture? How big are these holes?

If they’re small, get a good putty knife and some spackle. It works wonders!

Wall putty.

Also what is the material that is covering the opening made of?

Cover it with a rustic wall hanging!
Classy!

I’ll post one asap. They aren’t big at all, its just a bunch of different shapes. Stars and lines. The grill has been painted white and seems to be made of plaster.

Then use plaster of paris. Mix it up a little thick and mold it to fit.

Oops. Apparently rustic art forms remain unappreciated.

The issue with this query is: What is the issue? Is it causing a draft? Do you feel as though there might be a problem with pests emerging from the holes? Is your concern purely aesthetic, i.e. you just don’t like the pattern? Or does the thought of an indefinable black void in your quarters feel a little creepy?

Also, will your landlord allow you to smear plaster over his property?

Based on the information given, your options are limited. In terms of filling the holes, the options are putty, plaster, and (the extremely tacky, but quick, method of) filling the hole with aerosol packing foam. Just point and spray.

On the other hand, you could cover it with a piece of furniture or a wall-hanging such as a tapestry. unless it’s a “rustic” one, apparently