Mannanan doesn't do gates, he does "mists", and parts them.

From what I gather, Mannanan is muchly into things involving quantum physics, and as such, going from one reality to another (our world to the Otherworld) is not so much travelling across time or space, but between the probability potentials of the existence of each reality...sort of like how Schroedinger's Cat is both Dead and Alive at the same time until a consciouses choice is made to look into the box which collapses the state vector of one universe or the other...But Mannanan allows us to be be in both potential universes at the same time even after we have looked into the box.

Or to put it into a Star Trek Metaphor...it is like we were doing a regular transport, and due to his medling, we end up in the Mirror Universe (Otherworld) rather than our own. Which is very different than opening a doorway or gate and crossing into the Otherworld.

Another way to look at it is focusing your eyes on something near, rather than something far away...