RCT1 should be completed by both parties in advance of works. It is basically a sorting process to determine whether it is a subcontract or a form of employment that is being undertaken.
Fill the RCT1 in, get it signed, it will detail the value of contract / works, location, duration etc. You need to keep this for 6 years on your files.
It is a pain but you get in the hang of preparing these in advance, there is a fillable form on Revenue.ie so it you have nominated your subbies you can do all the paperwork neatly and simply print for signing.
I use the same set of subbies, in general, and can do this at the start of each project.
You do need to be careful accounting for the VAT reverse charge, it places much more dilligence on you.
If there is a C2 in place you need the RCT46/47, I'm unsure of which it is at moment, filled to get the payments card.
Don't make any payment without making the deduction unless you have a payments card. Even if a card comes in after the payment and all is well you can be fined €3000 for breaching the regulation.
As for contract, a purchase order confirming the quotation, duration of works and payment terms should be sufficient on small items.