Never ever be irreplaceable in your line of work.
If you are irreplaceable, it means no one can do the work that you do. This in turn means that your firm does not see anyone else who can deliver the quality that you can.
Such a perception can be a major setback to your career because your firm will find it difficult to promote you and fill your place with someone else.
Although many a times we strive hard to perform at our best, there are two different ways to look at it.
One: The firm will never fire you because you perform so well in your tasks.
Second: The firm will also NOT promote you, because they can't find your replacement so easily.
How to tackle this? I have a suggestion - a personal one ofcourse, so take it with a pill.
You need to maintain your skills in a such a manner that you offer a better deal than your peers, but at the same time ensure that you are not percieved as "the best" in that field.
It is a difficult task to do. So even after doing it all, if you can't bring your firm to realize that you too have a career, then I personally think it's time to re-evaluate your options.
Again take this harsh advice "with a pill".