The clearest red flags revolve around outsourced work and poor accountability. Several bad experiences trace back to companies that subcontract the actual install — you pay one firm, but a third party does the work, quality varies wildly, and when something's wrong you get finger-pointing instead of a fix. Some contracts even have wording that pushes responsibility for faults onto the subcontractor rather than the company you paid. Ask directly whether the installer uses their own staff, and be cautious if they won't give a straight answer.
Other warning signs: sloppy, visibly poor workmanship (untidy wiring, cables run anywhere), having to fight to get the company back to finish or fix the job, wrong components supplied versus what was quoted, and high-pressure sales or "rewards scheme" gimmicks that conveniently match the grant amount. Quality and aftercare matter enormously, because a solar system lives on your roof for decades and you want the installer reachable years later. A tidy, compliant, well-documented install from a registered company beats a keen price every time.
Protect yourself with a few habits: get three like-for-like quotes, check reviews and talk to local people who've actually used the company (asking specifically how problems were handled), confirm SEAI registration, and don't sign off the acceptance form or release the final payment until the work genuinely meets standard. If the wiring or finish looks shoddy at handover, you're entitled to refuse to accept it until it's put right. Reputation, own-crew installs, and solid aftercare are the things that separate a good installer from a lottery.