Sidesaddle depictions in the Epona artifacts are not the same as a modern side saddle where the woman rides with both legs on the same side facing forwards.

When Epona is depicted riding sidesaddle, the horse is shown moving forward at a steady pace (usually a trot or animated walk, although in the case of the Maryport Epona, it's a pace) and Epona is riding facing sideways. For example, a stone-bas Epona found at Tawern (see Tawern archaeological site for a picture, text is in German) depicts the horse goddess riding sidesaddle. There is a visible platform under her feet. Atlhough not all sidesaddle depictions show the platform under her feet, it provides a clue as to what the sidesaddle might have looked like.

I've often wondered if the sidesaddle would have been similar to the types used during the middle ages mainly because the sidesaddle had the woman facing to the side with her feet supported by a platform. Ilaria Veltri degli Ansari researched and reconstructed a medieval sidesaddle. The sidesaddle she created uses the same kind of platform for the woman's feet.

Excellent discussion of reconstructing a sidesaddle