"I am a 28 year-old Irish guy with an Honours Bachelors, Masters (History) and Postgraduate Diploma in Education - I am a qualified high school teacher in my native country. I have one year of high school teaching experience here in Ireland, along with eighteen months full-time ESL teaching as part of the EPIK programme in Korea.
I am interested in going back to Korea to teach at university level but I don't know the best way about applying for such a position, or even which specific university positions might be desirable. What kind of job and salary do you think I could realistically expect to get? And what working hours and vacation time would be entailed in such a position?"
You have a few things going against you. Namely, no university experience as well as not physically being in the country for interviews (I think...but not sure about this based on your email). Most unis want at least 2 years uni experience (but refer to my previous post) and in-person interviews. The other problem I can forsee is what exactly is a "Postgraduate Diploma?" If it's equivalent to a Masters Degree, then you're golden, if not, you'll have an even harder time finding a uni job.
You may luck out and find someone who will take you, without a Masters, without any uni experience and without being in country. But, it seems kind of unlikely to me. Your best bet would be to get yourself to Korea somehow and be available for interviews here. And then, just take what you can get. I'm not sure anyone looking for their first uni job in Korea can really afford to be picky for the first year or two.
If you do find a uni job, you could expect between 2-2.5/ month (plus housing included or around 300/month housing allowance). Hours are between 12-20/week and vacation is anywhere from 2 months-5 months. It just depends on the package offered and it varies so widely that it's hard to make an generalizations.