This is going to have interesting implications.
As a main contractor engaged in construction projects the obligation is on you to ensure tax compliance of all subcontractors through the C2 or RCT deduction system.
In the definition of this tax code the main contractor is responsible for the verification of the individual / company involved, confirmation of their tax numbers, application for tax deduction card, issuance of payments & deduction certificates (where applicable).
The records associated with this must be kept for 6 years.
The interesting point in this tax system is the penalty the main contractor faces for any payment made outside of the guidelines. You will be responsible for the RCT on top of any payment made. So if you pay 'Jim' €1000 for the supply and installation of a fireplace without tax deduction cards, tax clearance etc the main contractor is liable to pay an additional 35% to the revenue and to account for the VAT which is 13.5%.
Revenue are not interested in who is doing the work for cash per se, they will come after the main contractor, which is you, you issue the contracts. It is far easier for the Revenue to hit you for the additional €485 (from the example) than to find Jim with his van, possibly of no fixed abode, especially when they know where you live!!
Who said building was easy?