The only free and comprehensive online etymological dictionary of the Spanish language
ultra (Adverb) "beyond"
15th cent. From Latin ultra 'id.' From Proto-Italic *ol-tero- 'id.,' from Proto-Indo-European *h2ol-tero- "further." From *h2ol- "that" (see el (2), él).
Indo-European
Romance
Portuguese ultra, French outre, Italian oltra

An ancient comparative with *h2ol-tmHo- "furthest" (whence último).