Seth Godin's latest book talks about Linchpins: people inside a business that really are pivotal to its success. (That's an affiliate link for Amazon, just in case you need to pick up the book and read it).
Being indispensable is about delivering massive impact no matter where you are. It's much more of a characteristic - a mindset wrapped with skills and attributes - rather than the details and functions in a role. Indispensable people are the types that you can hand any project, put in nearly any role, issue a challenge to, and they simply make things happen by understanding what needs to get done and adapting their skills accordingly.
Being irreplaceable is the opposite. It's about being locked into a role because you're harboring finite knowledge, skills, or information that you can't or aren't willing to share with anyone else. Sometimes that's borne from insecurity. Other times it's a false sense that if you protect your sandbox so that only you know its secrets, you have job security for life.
If you can't be replaced, you can't be promoted. You can't grow your company or your profile, because it can't move forward without you. You can't grow, move on, do different things, expand your horizons. Being irreplaceable is actually a pretty crappy place to be in your career.
But indispensable? Hell yes.
Build your team to sail the ship without you. Teach them everything you know, and hire people smarter than you. Equip yourself to always bring something powerful, unique, and pivotal to your work, but make it a methodology, not a checklist that's unique to any one discipline.
And then carry that approach into everything you do.
The irreplaceable always, oddly, get replaced somehow. Think automation, outsourcing, downsizing.
The indispensable? They're they ultimate in adaptability, and thrive on whatever gets thrown at them next. Their skills and techniques are unique, and ever-evolving. Which means that few businesses can thrive for the long term without them.
Which are you?