Really good sausage are probably the least respected and underutilised of all meats in this country, especially considering the popularity of the breakfast sausage - which are a different product. I can get good sausages in my local market in Midleton from Fingal Ferguson (www.gubbeen.com) through Clodagh Mckenna's stall. Olivier, a dramatic French man who lives in West Cork also does fine sausages, although I can't really advise on where to get them, besides west Cork! At a more commercial level, Superquinn do a good range of sausages but being a Cork man and there being no Superquinn's in Cork, I haven't had found the time to check them out.
To be quite honest I'd love to hear of any local butchers doing good sausages. For really good sausages, first and foremost it's about taste, and only you can decide this. Other differences from the regular breakfast sausages are they are not always made from pork, they are usually flavoured with herbs, spices, garlic or even alcohol and they are -for no good reason I might add - usually bigger.