I would reiterate the above : DO NOT MESS AROUND WITH EARTHING/GROUNDING SYSTEMS!
In general, in most countries in Europe and the US etc the local rules when applied correctly are extremely safe. Where you get accidents tends to be where someone has made an addition or modification to a system without understanding what they're doing and this is why many countries, including Ireland, restrict such work to Registered Electrical Contractors.
It is, however, useful to understand roughly how these things work as at least you can understand why things like equipotential bonding of pipework is so important. Or, how to recognise a neutral fault.
The main advantage of TN-C-S is that you ensure that it provides a solid, reliable earth and will trip MCBs or burn out fuses quickly in the event of a fault because the earth is likely to be at zero potential (O volts relative to the live).
It requires a well maintained, quite complex local distribution system, as the supply neutral must be earthed correctly regularly along its route back to the transformer. So, in some cases in remote locations it may be unsuitable as the overhead local distribution lines may not be capable of doing that.
TN-C-S also has risks where there's a neutral fault that can potentially cause currents to flow across plumbing systems or underground piping. If you ever notice live pipework always call a REC.
TT, where the consumer relies only on their own earth electrodes, has the advantage of being very simple, but you can't really ensure that the earth is reliable without regular testing. Electrodes can become damaged / corroded by soil conditions and so on. Also depending on the impedance of the ground, you can have issues with poor earths which can result in breakers not tripping in fault situations or even live bonded metal work. That's why you use an RCD across the whole system at a higher rating (fire protection primarily) and 30mA shock / human protection.
TT earthing systems are encountered sometimes in temporary installations - power for construction sites, caravans, etc etc and also in some remote areas where the network neutral is unsuitable for TN-C-S.
In some countries TT is also preferred and used as the normal approach. France for example.
This is why you will always see a "Disjoncteur" installed by the power company at the meter.
Again, this is another reason why if you're doing DIY work in another country, you need to understand how the local systems work. If you apply your own familiar approach to a foreign system, you can end up either creating serious hazards or exposing yourself to legal issues.
There's also a third type of earthing called IT, this is where the neutral point of the transformer is basically isolated from earth. It's used in specialist circumstances like operating theatres and mining where continuity of supply is essential. You will not encounter this in a home / office environment in Ireland or the UK.
You do, however, have widespread use of local isolation transformers for handheld power tools and shaver sockets. But, they're localised rather than 'systems'.
