I know a bit about this area.
It comes down to contract law, the principle of utmost good faith, and the disclosure or otherwise of any material fact relevant to the proper assessment of the risk.
Modifications do not have to enhance performance in any way to increase a risk. For example the fitting of a fancy aftermarket stereo or high value items like alloy wheels may make the car more attractive to a potential thief.
I've often seen ads for cars like "Subaru 2.0 Turbo - non turbo on log book" - this is clearly an indication of potential non disclosure.
Any car that has been lowered or has uprated suspension, or has a roll cage fitted, or has engine mods such as performance air filters/exhaust, or has been chipped for extra power, etc. etc. will clearly be a non standard risk.
The bottomline is if you are in any doubt disclose. It's a basic principle of insurance law and it's up to the propser to do so, and not the insurer or gardai to catch them.
HTH.