There are antique fairs on regularly in Kilcoran Lodge Hotel, which is on your way from Cork to Cashel. Depending on the date you are arriving I would suggest looking up The Saturday supplement of The Examiner (www.examiner.ie) as it gives dates of auctions and sales in the Cork/Tipp area - look towards the back pages.
Shops tend to be more expensive and unless you are an expert (and perhaps you are!) I would go towards the auction route. However, there is one shop that I know of on Cook Street in Cork city where they seem to have some nice pieces and not too expensive.