If you check the SEAI grant section for insulation you will find a list of contractors that are approved.
These contractors can only get on this list if they have certification, insurance etc.
You are right to be concerned about the quality of the job. We had ours done by someone whose reputation seemed very good, recommened by friends etc and yet the quality of the job was so bad that we are looking at legal action. Fortunately this company is no longer in the trade, so you will not come across them.
Currently looking into myself. Have seen a number of external insulation jobs done in Switzerland (where cowboys are not allowed operate). They insulate right down to the foundations. Essential for underfloor heated houses (which are common in central europe due to sub zero for 3-4 months each year). If you don't insulate from the foundations, external blockwork temperature will track ground temperatures and you loose some of the benefit. Foam glass gravel is another option for loosefill from the foundation up to path level.