What could he have been disembarking from when he failed to check arms and was court marshalled?
As I read it he was Guard Commander on 10 May 1941 and was charged with:
1. being improperly dressed (guilty)
2. allowing members of his guard to be improperly dressed (not guilty)
3. failing to post his sentry at the appointed time (guilty)
4. failing to inspect the arms of his guard before dismounting (guilty)
I am guessing but I think "dismounting" here may be in the sense of standing down: as you would mount a guard, so you would dismount it.
Possibly there had been some sort of incident involving the accidental discharge of a firearm by one of the soldiers under his command.
and anyone know what they were doing in Northern Ireland?
There were many army bases and training camps in Northern Ireland. See
http://ww2ni.webs.com/countydownpart4.htm for information on Killyleagh Castle where the court-martial took place.