my husband did one by distance learning - torturous! He is a perfectionist which makes him a good proof reader but he found it nearly impossible to actually be happy enough with the exercise to return it. If you are anything like this try to get a live class or like him a different career!!
http://www.train4publishing.co.uk/ offer very good courses that are particularly respected by book publishers here. I don't know of any Irish proof-reading courses that are well-thought of amongst book publishers.
Cle ( the Irish book publishers association) occasionally run one or two day courses in their offices, but these are only open to members.
The type of proof-reading you want to do after the course (books, legal documents, magazines, specialist publications, etc) may also be a factor in the type of course you take.