I was watching the news on one of the Israeli channels, and a letter by the late Naomi Shemer (the most prolific Israeli lyricist) has laid to rest a controversy about the tune's origins.
The claim had been made that the melody was in fact from a Basque song, Joseba's Hair, performed by Paco Ibanez among others, a claim which Shemer had hotly denied.
Turns out she had used it unintentionaly, something she hadn't realised until it was brought up years later, and panicked.

http://www.haaretz.com/hasen/spages/572475.html