I also thought it was an offence to leave the scene of an accident without giving the other driver your details?
Two issues at work here. Firstly, strictly speaking, according to the rules of the road, your mother is in the wrong as she pulled out. The circumstances of the oncoming traffic doesnt matter - the license, insurance, tax etc. offences are independent fromt the accident. Yes, you could argue the other driver shouldnt be driving without these, but strictly legally speaking, these offences are a matter for the Gardai and fall outside the insurance claim.
Second issue - your mother wasnt informed. This is plain wrong. If it were me, I'd refer the matter to the insurance ombundsman. Normally if their is a threatened claim against you, you are informed and allowed put your case. Natural justice dictates that your mother should have been given a chance to respond to the allegation. There is no reason why her insurance bill should increase without her side of the story.
Also, if it were me in your mother's circumstances, because the other driver totally took the p...s, I would be insisting that the Gardai take a prosecution. The fact that she made an insurance claim is cast iron proof that she was driving illegally - no tax, insurance or valid license. Should be gimme for any Garda station wanting to bump up their detection figures. Dont take no for an answer if the Gardai cant be bothered - insist that they prosecute.