We need more information here really to give you a good answer:
Is the job a pitched roof (sloped) or flat? Is the repair just that or are you having your house re-roofed, in whole or part? Whether a pitched or flat roof your roofer should be able to show you a sample of the material he proposes to finish the job in: slate (natural or fibre cement), tile, roof membrane (modified bitumen torch on felt, rubber, pvc, asphalt etc). Ay roof material, including underlays (sarking felt etc) should have a current Irish Agrement Certificate stating that this product is suitable for use in this country and for the application proposed. Before employing any contractor to do any major work on your home ensure that they have adequate public liability and employers liability insurances in place to cover their activity. Ask for references of previous work completed and check these out. Never pay cash up front except where you may be asked to pay for some materials that are a special order; in this case ensure that the materials are in your name and delivered to your address. Never pay completely for the job until you are staified with the work and that it is complete. I'm not sure how much use a "written" guarantee is, unless it is underwritten by an insurance company. One last thing , never employ someone who cold calls on your door touting for work- professional tradespeople don't knock on doors for work, they don't need to.