Hi Rizzo,
I think you are on track regarding the warrant officer detail.
There's a remarkably similar trio photo on this site (link below).
I believe is was because at that time, only the Sergeant Major and the Band Master were appointed warrant officers; and the regimental school master had the same status, although wasn't badged as such. Army Schoolmasters wore the Queen Victoria crown as a cap badge. (anyone own one?)
The major difference with these photos and Toby's picture of the Northamptonshire Regt WOs and Schoolmaster is that the sergeants and WOs on these pictures are not wearing the infantry sash - this is because they were a rifle regiment and not infantry of the line, (a tradition maintained today by The Rifles and the Royal Gurkha Rifles).
http://www.lennonwylie.co.uk/RoyalIrishRifles2nd.htm