Until the arrival of Sources, the search for non-published archival sources for Ireland was dependent on the famed Hayes Catalogue, a multivolume set consisting of a collated card catalog that had been photographed, printed, and bound in a large folio-sized form. Needless to say, this was early 20th century technology at its finest, but hardly conducive to today's research by multiple keyword entry points. Moreover, for overseas researchers, unless you were lucky enough to have access to a library who had purchased these volumes, you could plan on several weeks at the start of your research trip trolling through the Hayes volumes when you would have preferred to have your list of manuscripts to consult ready and assembled.
Sources, like many of the newest online catalogs to emerge in recent years (say, the new WorldCat and the British Library database), is formatted around a simple keyword search with the ability to filter instantaneously by selecting subcategories on the margins of the produced results. This, too, has been helpful in that it makes choices about filtering more informed--you can see immediately how many results, for example, would be provided for each filter applied. It also enables you to stumble on tangential sources because it provides a list of potential subject categories related to the results, rather than forcing you to guess subjects, genres, or tags that might bring up such sources. This mitigates a major problem with today's keyword searching, namely that it is so directed that the researcher may no longer stumble upon useful items along their way to the item being sought.
But perhaps most interesting given the worldwide scope of Irish studies is the spatial mapping of archives and the size of their holdings. Here, one learns that wherever one's location, it is likely that there is an archive nearby with some Irish-related material to consult.