Is it possible that the Dornish people’s long abiding love for spears rather than swords, axes, bows or any other weapon developed because of the importance of a spearwall in stopping the massed cavalry charges used by their ancient enemies of the Reach?

Well, the Dornish spearfighting traditions were largely a Rhoynish import, as the Rhoynish warriors had fought with spear and shield for thousands of years. 

From what we’ve read, the Dornish seem to emphasize highly mobile cavalry that act either as javelineers (similar to the Numidian cavalry who formed the core of Hannibal’s strength in the Punic Wars) or as lancers, so I don’t know to what extent the Dornish use spear-infantry.

If they do, it seems to be less about spearwalls per se – Nymeria speaks of “the spears would be falling thick as rain upon the marches now”, and we see throwing spears used when Areo Hotah ambushes the conspiracy. So maybe the Dornish use Zulu tactics?