Is it necessary to switch on the camera to apply in Ireland?

It depends on the university and course. Most applications to Irish universities are document-based and do not require your camera to be on. However, some programs may ask for an online interview after you apply - if so, you will need to have your camera on during the interview.

For student visa applications to Ireland, interviews are not standard and usually aren't required, so having your camera on is not necessary for the visa process.