Tá fáilte romhat, a Sakutama. I just love it when people from countries (especially as far away as Japan) other than Ireland show an interest in the language. I certainly wish you great luck with all your studies. It can be slow going if you have no native speaker near you, but it is still well worth the effort. If you speak Japanese then you may be a quick study when it comes to syntax and other traits of Gaeilge.
I started out with Teach Yourself Irish and progressed to Rosetta Stone Irish (which is well worth the money as a tool to get you speaking but is not quite enough on its own without other practice methods) as well as just using current online materials, literature, and newspapers as Gaeilge to learn common phrases. Online there are two dictionaries that I use very often for quick reference: http://www.potafocal.com and http://www.focal.ie . I also keep a paper pocket dictionary on hand just in case I need more clarification or one of the terms is perhaps not on the online dictionaries.
And of course, this forum is a great wealth of information and assistance should you ever get stumped on a word or phrase or grammar rule.
Fáilte romhat arís, a Sakutama, agus go n-éirí leat! (Welcome again, Sakutama, and good luck!)