Depends on the type of work? And also if you have a degree?
Good thing about the WHV was that you could work quite unrestrictedly and allowed you to work most jobs as long as it didn't violate immigration laws (like doing illegal work). But usually most would work the backpacker jobs like Bartending, Waiting/Waitressing, Hotel/Hostel and some also teach English while on the WHV
Going back there now you won't be able to do those type of low skilled work like Bartending etc but you can teach English and you'll need a degree. Teaching English requires any type of degree but more specialised jobs will require more specialised degrees.
Basically, if you have a degree, you can try get work in Japan. Most teach English but if you can find specialised work relating to another type of degree you currently have then thats good too, but you will most likely require high level of Japanese to work these jobs (besides teaching English)
I think with the Yen low at the moment it's better if you actually work in Japan as you can pay Irish bills & stuff easier. Back when I went it was 1 euro to 133 yen, which was good if you were travelling to Japan it meant more money to spend. But if you were living there and wanted to come back to Ireland you had to use more yen just to get your euro's worth. Now it's the opposite and it's harder for tourists to spend money but would be easier for those working in Japan to get their Euro's if they wanted to exchange it.