Domestic electric covers any electrical appliances and connections back to the fuse board in a home setting. Beyond the fuse board is covered by the company supplying electricity and cannot be tampered with.
Homeowners are allowed do certain tasks themselves according to the regulations1.
Lights
Lights typically use 2-core 1mm or (more typically) 1.5mm (neutral and live) and are wired with the live wire split inline by a switch. More modern wiring uses 3-core (live, neutral and earth/CPC) to switches to allow smart switches to be installed. Any metallic surfaces or those that are not insulated (for example being plastic) must have an earth connection.
Fans
Fans use a 4-core wiring (Neutral [Grey], Permanent Live [Brown], Switched Live [Black] and Earth [bare wire]). They should be wired with a fan isolation switch2.
Sockets
Sockets use 3-core 2.5mm cable (live, neutral and earth) and are wired in a ring. You can spur from an exisiting socket but must never do a spur from a spur. Sockets themselves typically have 13A fuses - meaning two plugs can draw maximum 26A. The ring itself is usually fused to 30A or 32A.
Footnotes
1 https://www.hsa.ie/eng/topics/electricity/inspection_and_testing/works_needing_certification/
2 How to Wire an Extractor Fan or a Second Light From a Ceiling Rose and Pendan…