Sheila does look like a good candidate for Disproven Existence, and some additional work is needed. Uncertain Existence is the correct category until the work is completed.
First, I would suggest that you add a tag for Ireland on the question because her legend is a part of Irish history that impacts on genealogies. It may be useful for the Ireland Project to create a category for multiple stories of Irish nobility dispossessed and exiled by the English.
Second, I did a slight edit to the profile. A person who never existed can't really have a biography, so the discussion needs to be under research notes. I looked up the Wikipedia article which gives an extensive summary not only of the legend but of how it developed, so I linked a sentence about that under Research Notes. You may want to look at the other articles that are cited. We don't need to list all the places the legend appeared, but it's useful to identify where it first appeared.
Third, a person who never existed cannot have parents, spouses or children, so the spouse and child currently attached need to be reviewed and detached. They should be linked in the Research Notes, i.e. "In many versions of the legend, Sheila is linked to (Husband)" -- with a link. It's possible that the husband is also legendary, or may be real. But that should be addressed before you're finished with Sheila. Same with the child, who may well be real.
Fourth, you can expect there may be some good discussion here on G2G. Before you're finished with Sheila, her narrative needs to be re-written to reflect the salient points of the discussion. Then there will be a profile we can all be proud of which will give some truth to those who visit WikiTree!
Then the Disproven Existence Project can take over the profile!
