When the construction stage of most projects is reached a Commencement Notice must be served in advance. In the case of domestic projects the client has the choice of ‘opting in’ or ‘opting out’ of Assigned Certification. 

The duties of the Assigned Certifier were brought into Law in Ireland in 2014 with SI.9. This role is explained in the following document issued by our institute the RIAI. RIAI BC(A)R Client Guidance Note 2016

This role can be a separate appointment (i.e. doesn’t have to be the architect). If the client decides to 'opt out' it means that the client will be responsible for certification of the development in lieu of a designated 'Assigned Certifier'. The fee for Assigned Certification is separate to standard architectural services.

In addition to the above mentioned BC(A)R Guidance Note see this link also. Also here is an article in the Irish Times that gives some pros & cons from more of layman’s perspective.

Its 50/50 re the amount of clients that opt in v opt out. From our point of view its more work but it’s also more liability!