If it is discovered immediately there is a possibility that the porting can be reversed within 24hours, depending on the carrier and type of port.
In general though, you will need to request the old carrier to port the number back to them which is a completely new port request and can take up to 10 working days.
This situation should only come up if you have completed the porting request without understanding the implications of moving your numbers. IT.ie
is very experienced and can advise you on what you should and should not do.
If you are uncertain at any stage in the porting process please contact our firstname.lastname@example.org.