When painting the inside and outside of the brick fireplace you need to use a high temp heat resistant paint that is made for painting in the interior.
Use High Temp interior paint and use a roller to get a good look.
You could also use high temperature paint in an spray can as well where you just spray it on.
Be sure to clean any ashes out of the fireplace and vacuum and wipe down the fireplace and scrape off any old paint including flaking paint in the fireplace to get a good paint job.