Do you know what you'll be looking for? Can you identify poor workmanship? Do you know what a reasonable standard of finish is? Are you familiar with Irish building regulations and conventional construction technology and practices? If in any doubt I'd say pay a professional to do the snag list, unless you know a friend or family member who's in the know. A good snagger should check the house, inside and out, room by room and investigate the attic. While no guarantee a thorough snag list is the final "stick" in your dealings with any builder- you shouldn't close until all reasonable items have been addressed. Should you decide to tackle the job yourself you should check out the Homebond manual which gives a sample list of typical items to be checked; this is a useful starting point.