A fully mechanised system will rely on a centralised unit and ducting to each room of the house. There are many different types of mechanised systems but Mechanical Ventilation with Heat Recovery (MVHR) has become a go-to solution for self-builders for its efficiency, as it recycles heat from within the home through a heat exchanger. It is a fully mechanised system and may offer some form of cooling in summer, but this depends on the model. MVHR cannot act as an air conditioning device because it is a system designed to optimise air circulation.
Other options that aren’t fully mechanised include demand control ventilation (DCV) which also has a centralised fan but relies on humidity sensor vents in the rooms to extract stale air. Meanwhile positive input ventilation is an age-old system that relies on the passive stack effect but may have trouble working in some areas with high winds. There are variants of all of these.
Both natural ventilation and fully mechanised ventilation systems, and hybrid solutions in between, must be designed by a professional. The ventilation system must be designed for your specific home to get the ventilation system to work as intended. For example creating a voids in walls and ceilings will allow for the larger sized ducts which reduce the noise of the system and improves its overall efficiency due to lower resistance.
In ROI the building regulations stipulate you must have the system both designed and commissioned (tested) by an independent energy assessor. In NI for fixed building ventilation systems which require commissioning, the regulations do not specify who does it but it must be done in accordance with the procedures given in the Domestic Ventilation Compliance Guide as specified in the Building Regulations, so it is advisable to at the very least get the company you are buying the kit from to take on these roles to maximise efficiencies.