Yes, you can co-witness holo sights with iron sights. Most of the time, at least.
There are two types of co-witnessing:
Absolute co-witnessing
Lower one-third co-witnessing
If you look through the iron sights and optic at the same time and the holographic dot is on top of the front sight post, then your sights are absolutely co-witnessed.
Lower one-third co-witnessing is when you look through the holo sight so the reticle is in the center of the glass, and the front sight post appears on the lower portion of the glass.
Some people prefer absolute co-witnessing so you don’t have to adjust your head position if you want to use the irons. Flip-up irons can help keep your sight picture from getting too busy.
EOTech models with built-in quick-detach mounts, such as the EXPS3, are designed for lower one-third co-witnessing with iron sights. The other models mount lower on the rail and so will give you an absolute co-witness sight picture.
Note that this is AR-15 specific. An EOTech 512 that has absolute co-witness on an AR-15 will not co-witness at all on a C308.