Does the use of zopiclone every night (7.5 mg) for several years cause irreversible IQ reduction?

Highly unlikely. I'm not aware of any studies to support lowered IQ from Zopiclone use. It's recommended for short term use for insomnia. Talk to your prescribing doctor about any concerns re your cognitive ability, concentration, etc and consider taking a break to see if you notice any difference. Also be honest about any other meds, alcohol or substances you ingest which could be making you worry.