If you’re irreplaceable at work, chances are you did some part of your job bad or they overlooked you.
Not because of your skill, but because you didn’t allow the structure to grow, making it dependable on you, which is bad (you know, holidays, you want them , right?).
The fastest way to fix this in case you’re not understanding the issue is to broke the system for a while.
Replace you, remove you from the work you’ve been doing and let you work on something else.
This will create disasters on the short term, but in the long term it’ll allow you to have a better workplace.