An e in front of an o changes the o to an ó sound. No need for a fada on the o. Beo, ceo, deo, eo, feo. Like an a after an i changes the i to an í sound, Niamh, Cian, Dia, Liam, bia, sliabh, mian, liath. Similarly, an a after a u changes the u to a ú sound. Slua, crua, rua, luath.
Fadas are not optional extras. Órla/orla, Seán/sean, Éire/eire, bríste/briste, féar/fear, í/i, sá/sa, dó/do, Dé/de, dí/di, cáca/caca. Quite the difference between a piece of cáca and a piece of caca.