In both jurisdictions, if you are doing all the work yourself, i.e. a full DIY project, the regulations do not apply. This scenario is highly unlikely as you would almost always need specialist contractors such as electricians, plumbers, etc.
In both ROI and NI, projects that involve more than one contractor or are scheduled to last longer than 30 days or involve a particular risk, the owner must appoint project supervisors for the health and safety aspects and notify the relevant authorities: you need a supervisor for the design process (in ROI Project Supervisor Design Stage or PSDS, and in NI they are called the Principal Design or PD) and another for the construction stage (called the Principal Contractor or PC in NI and in ROI, Project Supervisor Construction Stage or PSCS). The PSCS/PC role may be taken on by the main contractor.
Whilst it is possible to undertake the role of project manager and fulfil both roles, be aware that you must be “competent” to do so. You will also probably need the help of a H&S professional to help you with the documentation and site specific aspects.