Came across this question while looking for the answer to it myself. Since then, I've figured it out (at least for my situation)...
It looks like Irrelevant shows up when you have an issue type in your filter that doesn't have the field you are grouping by. So, in your case, you must have 17 issues of an issue type where the "Raised In Versions (all)" custom field isn't available.
There really isn't a great way to get that list of "Irrelevant issues" in JIRA (since you can't use the "Raised In Versions (all)" in your search), although you could try adding an Issue Type filter to your query and exclude any issue types where you know that field is available.
Hope this helps!
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