From the Library Technology Guides, October 22, 2010 reference, and includes other contributors and notes about the Koha Integrated Library Software Open Source code release.
Here is the link for the Koha 3.2.0 announcement, reference to Darrell Ulm as code contributor.
Link shows past work, much has moved to GitHub for Darrell Ulm.
For work that was done further back before GitHub, some information is at: Darrell Ulm, Open Source Software profile at OpenHub, which is an interesting site to show open source contributions.
The link shown on this page includes reference to Darrell Ulm, on 4/2/2008, becomes the 57th code committer to Koha. And here is the History of Koha, and Open Source Integrated Library System.
This is the announcement link for Koha version 3.2.0 from 2010:
Koha 3.2.0 Open Source ILS Announcement
which has source code contributed by Darrell Ulm, October 22, 2010.
There were some nice enhancements for the 3.2 version compared to previous versions and was when the Koha version 3 started working better overall.