For The Tuatha, A Dagda type pretender would be absolutely incredible.
For Eriu, if it is going to be based on Red Branch lore, you will have no shortage of national heroes for that nation
Cuchulainn
Cathbad the Druid <-----Please Please!
Emer
Etaine
Deirdre <-- very good choice for a heroine unit #awe etc.
Naoise