Stamp 4 after Critical Skills

I would be starting my new job in Dublin on the 1st of Feb, 2016. The Critical Skills permit was issued from 1st December 2015. Hence I would still be within the 90 day limit to start a job after the permit is issued.

On the DJEI website (renewals- Critical Skills EP) it states that:

If you do not satisfy the qualifying criteria you will be issued with a Stamp 1 by the Garda National Immigration Bureau and you will still be required to hold an employment permit in order to work in the State. - See more at: https://www.djei.ie/en/What-We-Do/Jobs- ... C7LsS.dpuf

Can some one kindly explain how the above scenario would work? would i need to reapply for a CSEP to complete 2 years ?

I have heard you are required to be resident for 21 months for stamp 4 to be granted. I would be required to go to the USA for training etc ( at least 4 weeks ) during my employment. Would the time spent on official visits to the US while resident in ireland be counted as absence towards the 21 month period?

I have been trying to ring up GNIB for clarrification with no success.

Any help would be greatly appreciated

Kind regards,