Dear Linda:
Retirement was much an individual matter; initial enlistments for the cavalry were for 12 years, and many of the men re-enlisted at the end of that time. Conditions of army life (housing, workload, diet) wore men out before their time, so that some relatively young men (late 30s to early 40's or even younger) were 'discharged as useless' based on their inability to fulfill the necessary duties. Men were also allowed to purchase a discharge if they wanted out of the army before the end of their service commitment.
As the movements of the Lancers are not a particular specialty of mine, I had to cheat and look it up, but they do not appear to have spent much time in Ireland around the time of the war. If you want to check out more specifics, go to "Google Book Search", do an "advanced book search", use "17th Lancers" as your search term and do your search for a "full view only." Two of the most authoritative books on the history of the 17th Lancers are available there to download in their entirety as PDF files: Fortescue's "A History of the 17th Lancers" and Parry's "The Death or Glory Boys."
Farrier-Major or Farrier-Sergeant was the NCO in charge of the farriers that tended to the horses to include blacksmith duties.