Origin

Star Trek Canon Special Note
Not everything listed in the origin is Canon.  That is because only television shows and movies are considered canon.  This being a comic website there are many things here that are not canon.  So unless it is contradicted in film it should be listed as part of the character biography.  
 
Dermot Cusack was the Yeoman under Captain Christopher Pike.  Pike made sure the Mr. Cusack followed him when he took command of the U.S.S. Enterprise on it's five year mission.  Pike said he was the best Yeoman in Starfleet.
 
Dermont is Irish and he fancied himself as a miracle Much like the famous Montgomery Scott who later served on the U.S.S. Enterprise as well.  worker and had a far reaching reputation of being roguish.  This was noted even by upper command notably Robert April.  Even Pike felt it fit his character.
 
Dermot was killed on Rigel VII when he was stabbed in the back after he saved his fellow crewmen after they were ambushed by Kaylar warrior.  The death of Durmot deeply effected Captain Pike and he almost left the Starfleet because of it.

Creation

The character of Dermot was created due to a line in the original pilot that had Christopher Pike claiming his Yeoman had died on Rigel VII.  In the series Star Trek: The Early Voyages some of the stories before Rigel VII were flushed out giving Dermot a lager role in the Star Trek Universe.