This method is very small scale, irrigating a single tree (I saw it for pomegranites in Iran), but appears effective and requires only pottery as a technology base.
Make a spherical pot, roughly 40 cm diameter with thick walls (about 5 cm) to withstand roots seeking water and a long (30 cm) narrow tubular neck. Glaze most of it but leave a large circular area on one side unglazed. Bury it with only a bit of neck exposed next to your tree with the unglazed part toward the tree. Fill with water and cover the opening; top up as needed. The water seeps out slowly toward the tree and, since it is not on the surface, evaporation loss is almost zero.