We presume that Plotinus' encounter with the One had its subjective correlative, its phenomenological aspect. His was what we might call a "mystical experience." On the other hand, if we take "to become one with the divine" in its strictest sense, then perhaps such a thorough identification with the One, resulting in a radical de-identification and separation from the physical, will leave no trace on the conscious mind. Plotinus himself wrote that the soul when unified cannot distinguish itself from the One and so can neither recognize nor relate the event. Unification gives rise, not to knowledge, but to a presence beyond knowing (6.9.3-4).
Plotinus evinced no interest in trying to describe his experience (nor did Porphyry after his vision; nor Aquinas later). In any case, whether or not unification has a phenomenological correlate, the subjective aspect is secondary, derivative, beside the point. This is the premodern attitude. For modern man the subjective experience is everything. For the premodern the object of the experience is all that really matters.
[Mark Anderson, Pure: Modernity, Philosophy, and the One. Sophia Perennis (San Rafael, CA: 2009) 26-27.] The reference, of course, is to the Enneads.