Moytura: Mag Mell Expansion adds:
2 more double-sided map/setup cards
1 new deity tile (that optionally replaces one of the 4 core deity tiles). The deity is Lugh, God of Mischief, and he lets you reposition the non-player enemy units on the board which can allow for some interesting strategies.
3 new special enemy clans that each comes with their own set of units, cards, and tokens. They include some new token types you don’t see in the base game, such as permanent influence tokens (gotta try to prevent those from appearing before they stick forever), bonus scoring tokens (for whoever controls the space), and portal tokens (these are particularly nasty for letting the enemy overrun a region).