Immersion has 4 wires instead of 3

You hope

If you did the work yourself it may be heating but not setup right or you may have left a timebomb with poor connections

It's Certainly what I would consider a grey area. When someone is asking basic questions about such a critical item then they have no business messing with it.

You asserted that the OP "by not calling an electrician … have broken the law and undoubtedly invalidated your house insurance if it causes a fire." That is a bold statement, that doesn't stand up to scrutiny. It is a like for like replacement and is most likely categorised as a minor work. (I say 'most likely' as the authors of the applicable documents could have been more clear.)
The issue of a fire is different. There is a requirement for anybody doing electrical work to be competent (again, no definitive standard set). If the fire could be shown to be due to a poor quality installation, the installer could be held liable. If the installer was, and remains, RECI registered then they should have adequate liability insurance to cover this.
You also asserted that "There are some things you are simply not allowed to do unless you are qualified, and anything to do with water heating is one of them." Other posters on Boards.ie have stated that they got written confirmation that replacing an electric shower is considered minor works. If true, and I fully expect that it is, this point also fails scrutiny.

Look, there is a broad consensus that the OP's lack of basic knowledge meant that they shouldn't be doing this work. That is a long way from suggesting that only a RECI registered contractor should do minor electrical works. However it is certainly wise to recommend caution, as other tradespeople may not have liability insurance should issues arise.

You can add a simple timer unit to your present set up - it's very simple to operate just press the button once for 15 minutes, press again for 30 minutes, again for 1 hour and again for 2 hours. It will switch off automatically when the pre-set time is reached. It eliminates the worry of careless teenagers and some adults😊 leaving the immersion on 27/7 The minor down side is that you'll have to reset it again manually next time you wish to use it. Of course there are more expensive & complicated 7 day timers available.

That unit requires an optional wifi dongle to allow you to remotely set timers. According to that website the unit and the dongle come to nearly €220 when you add on the vat.

That seems a little expensive for what is basically a timer you control with your phone. Is it really going to save you that much money.

That timers not bad as a standalone control.

It's a successor to earlier timers designed for dual-tarriff and also suits the newer multi-tarriffs

I fitted loads of the earlier horstmann models . I'm dubious about the need for bluetooth, apps and energy monitor for an immersion heater.