Your post is quite brief. Do you want to change it? We moved house in Cork some years ago. We wanted to keep the same number but, despite only moving about half a mile, we had to change number as our area was covered by a different exchange. (My point is that if you are staying in the same area, you may be able to hang on to the original number).
As far as I can remember, it took little or no time (a day or two) and I would expect that it would take even less time nowadays.
If, for some reason, you have to get it changed, (and maybe you're not moving house at all), I presume a call to eircom would answer your question. That's if you are an eircom customer. It might be different if you deal with another phone company.