One of the disadvantage of taking cash....and I won't go on re risk your family is taking re being uninsured and an accident was to happen with a child in your care, never mind taking fees off registered childminders and purpose built centres who despite societal thinking they are NOT turning huge profits, the large chains are but not the small independents. Already the rot has set in with montessoris and creches closing down. Parents are also at moment getting fee increases in care to cover cost of ecce, much expected in light of second year announced for Sept 2016. Another point is your landlord property is also uninsured due to your unregistered childminding service.
It's very easy register with revenue and you are well aware of the 15k tax excemption. I'm assuming you claim Home Carer Tax Credit, this is affected depending on her earnings.
As for banks, they require accounts for at least last two years but could be three. A cash in hand childminder fees can appear good but factor in insurances, 500euro prsi and tax if over 15k, equipment, food if supplied, art supplies, toys, curriculum, requirements/house adaptions to achieve registration by your county childcare committe or Tusla etc, training and strict ratios all registered childminders will say it's a tough environment made worse by unfair competition.
Your county childcare committe and Childminding Ireland are great for information on becoming a professional childminder.