Well Saga is a dark ages game so I wouldn’t expect to be able to play against a dark ages army with your Romans or Celts. However, you could certainly play Romans vs Celts using Saga, either using a home -made (whether by you or somebody else) battle board, or choosing the battle board and troop selections from the Saga army that best matches each army. For example you could use the Viking battle board for Celts and the Scots battle board for Romans.
There is a section on the Studio Tomahawk forum for discussion of new factions, and players have contributed their own designed battle boards. There is one for Celts, but not EIR I think.