I have not used celery, but I do recall reading about it sometime in the past. Celery has a very high water content, but the fibrous strings would be undesirable in soap, so grinding it up to a very fine liquid, then straining out the fiber would be best. Then use it in place of water in your lye solution.
Here is one link about a celery soap. Although, I am pretty sure the green tint one may get from celery fades over time, as most botanicals do, so I would add another colorant to keep the green color if desired.
I also saw it used in a body scrub, if you want to find additional uses for celery.
Here is a link to a chapter in an ebook with some recipes using celery. I have never tried it myself. I do make body scrubs sometimes, but don't use them very often.
Me, I prefer to use it making SOUP. I make soup from scratch all the time (not a big fan of canned soups) and celery is an important ingredient in soup, for me anyway.