photo ID - UK to ROI
The CTA (common travel area) between the UK and ROI dispenses with the requirement for Passport; this is the case since 1922.
Security requirements has nothing to do with it.
A valid - i.e. current - Driving licence with photo is sufficient.
For the record the following extract from the Ryanair site says it all:
'A valid European Economic Area (EEA) driving licence with photo, presented by a passenger whose place of birth is within the EEA is acceptable only for travel on internal flights within the UK, internal flights within Italy and UK-Republic of Ireland-UK'.