There are two Roud Indexes; the Roud Folksong Index, and the Roud Broadside Index, which are both accessible online through the VWML Online in the Vaughan Williams Memorial Library section of the EFDSS website. If you click on the 'Search Indexes' button, you will get a dropdown menu leading to Folksong Indexes, with a forther option to choose the Folksong Index or the Broadside Index, or to cross search both. The VWML home page explains the resources available, and there is also a helpful guide to Searching the indexes.
You can read more about using the indexes in the Enthusiasms section of the Musical Traditions website; just click on article #29. The article relates to a time before the indexes were avalable online, but the main principles are still the same.
Steve Roud, who compiles the indexes, is about to overhaul them, and perhaps when he has done so it may be useful to add a guide on Mudcat to using the indexes.
Happy hunting! and be warned that there are many of us who became immersed in the indexes years ago, and have never been the same since!