Might not be a good move replacing copper cylinder. There are grants available to upgrade oil or gas boilers which obviate the need for a copper cylinder. There are terms and conditions though.
The grants are administered by Sustainable Energy Ireland, (google it to find site as I can't post links as Im new member here).
But the terms and conditions do not apply for individual items but for "heating controls", not just boilers on their own but several other components, eg new room thermostat, motorized valves, cylinder thermostat etc which adds up to a lot of work and money that home owner has to pay for.
Also grants Only apply to properties built Before 2006! If built after that- no grant! But on balance of probabilities the posters property was built before 2006.
Your suggested that person get new boiler etc instead of replacing cylinder is a lot of money and work extra pipes, other components etc is a lot of hassle and money upfront.
The specific boiler to do away with water cylinder is "combination boiler", but then again if thats installed etc, in the years to come when/if that boiler breaks down, the person has NO hot water at all! No way to heat water separately by immersion cylinder electric immersion heater switch.
People should get all the facts first before embarking on any works to their homes.
My advice in this case is go ahead and replace copper cylinder- the most cost effective solution all round. But even better is for people to phone up plumbers/builders providers in their local area/national chain of companies eg Chadwicks, Brooks Thomas, Heatmerchants etc and Ask the prices of materials and then suss out tradesmen when they phone them up to get prices to do works in their homes.
Btw theres stainless steel cylinders aswell as copper cylinders, so theres the option, as s/steel cylinders are apparently less prone to rust than copper cylinders. People should get as much facts as possible.