How to install a doorway through a stone wall where a barrel ceiling is on one side

We have an old stone built cottage. We have a garage we are converting to a bathroom. The garage roof is a barrel brick ceiling. We need to break through from the house to the garage but need to know how to approach putting a lintel in where the roof is barrelled as to get the door height we would need to remove some bricks from the barrel ? any advice or links to articles/you tube videos would be appreciated. Thank you.