Several of my clients use the Communardo User Profiles for Jira app. (Disclaimer: I have no affiliation with Communardo nor do I benefit from their products).
One of the shortcomings of this plugin is the lack of a REST API to retrieve user profile information. I have written a REST API that can be used for this purpose. It requires ScriptRunner as well as the User Profiles for Jira app. I am offering access to this code for free - all you need to do is send an email to derek.fields@rightstar.com requesting access to the Bitbucket repository.
Please note that I am not actively supporting this, but I will provide updates if requested.
Installation and usage is documented in the README
@Martin Gypser Please send me an email directly to derek.fields@rightstar.com and I will grant you access.