About the sort order: Tunes that are tied with the same number of people playing them are then ranked with the oldest tunes first, based on the post-1900 publication histories of the tune, as documented in irishtune.info.
Caveat: The accuracy of these "session" statistics depend on how carefully each contributor was about entering his or her repertoire. There are known cases when users have hastily grabbed at a wrong tune only because it happens to have the same title as the intended tune, without bothering to read more closely. This behavior should only have a minor effect on these "most popular" lists.