Considering that the ancient kingdom of Zhang Zhung is a part of what we now know as Tibet, and that some of the most ancient teachers and adepts came from Uddiyana (or Urgyen), it becomes clear that a "first" dispersion of Buddhist Tantra or Tantric Buddhism northward, into these Himalayan regions, must have occurred long before the officially recognized first diffusion (after 750) to Tibet.
The difficulty in tracing this original impulse (from India to the Tibetan regions) is not only one of missing historical records, but also one of conflicting views as to exactly which teachings are "Buddhist", which "Tantric" and which "Bön".
Leaving such problems aside for a moment, it appears that Mahayana teachings (developing since the 1st century BCE) reached the Swat Valley (now Pakistan), Afghanistan, Zhang Zhung (now Tibet and north-western India) and other Himalayan regions only a few centuries after the historical Gautama Buddha (563-483 BCE) held his sermons in Central and North Eastern India. At roughly the same time in which Mahayana truly developed, the first few centuries CE, also the hitherto not organized nor written teachings of Tantra started to blossom, though they reached full maturity only in the 4th and 5th centuries.
The north- and westbound dispersion of Buddhism was no organized activity. Rather, it occurred more or less randomly by way of traveling ngakpas and yoginis, siddhas and scholars (mainly from India but also from China) who taught what they knew and had experienced, regardless of whether or not their techniques were Tantric and Buddhist; and also without any express aim to establish or found specific lineages or schools.
Among those who were active during this period and who later became famous - while others were lost in unrecorded history - are the likes of Prahevajra (Garab Dorje), Sri Singha, Jnanasutra, Manjusrimitra and Tönpa Shenrab; teachers without exact historical dating yet who are regarded as the very founders of the Bön, Dzogchen and Nyingtig lineages - a curious yet potent mixture that was to be spread further by Padmasambhava (see stage 2).
Now concerning the actual arrival of Buddhist texts in Tibet, this seems to have happened first during the reigns of king Lathori Nyentsen (circa 460) and king Songtsen Gampo (d. 650), although their impact was limited in the absence of skillful translators and commentators.