Steve,
They would have been garrison-ing and/or depot-ing!
Generally British Army regiments consisted of two battalions, designated "1st" and "2nd". While one was on active service abroad the other would be stationed at home, on "Garrison" duties, being moved around frequently to keep interest and fitness up. In times of severe crisis both battalions might serve abroad simultaneously.
[During 1808-1821 "Home" meant all of Britain and Ireland, together with their associated islands - especially the strategically important Channel Islands.]
Each battalion consisted of 10 companies, consisting of 80-100 men.
Two of the companies were designated "Depot", responsible for provisioning the regiment with supplies - including new recruits.
[Recruits were given a bonus to sign up, and paid 1s per day.
Standard service term was 12 years, with expectation of 6 "safe" years at home and 6 "interesting" years abroad "with the colours".
Men could be transferred between the two battalions of the regiment, usually to maintain the strength of the active battalion, or out to other regiments, sometimes at their own request.]
Most of these depot companies were based at home, even if the main body of their regiment was in action in foreign parts. The 8 active companies of a battalion on garrison duties could be serving in multiple locations at one time, frequently separate from the depot companies.
When in Ireland it is likely that the 35th Foot would have concentrated its recruiting efforts in Co. Antrim, especially Belfast - the town of its creation!
In 1693 the 3rd Earl of Donegall (CHICHESTER) was commissioned by William III (William Henry, Prince of Orange) to raise a regiment consisting exclusively of Belfast men. They were given permission to use the orange colour on their badges - hence their later nickname "The Orange Lillies". The were also known as the "Old Belfast" regiment.
They had an interesting history, losing their founding Colonel at Barcelona in 1705 and nearly being wiped out in 1707 at Almansa. In North America in 1757, they had a hair-raising time escaping from Fort William Henry along with the last of the Mohicans, then helped Wolfe take Quebec. They fought at Boston and New York during 1775-78 then soaked up the sun in the West Indies for 7 years. They won battle honours at Maida in Sicily in 1806, but their ranks were depleted.
They adopted the name "Royal Sussex" in 1804, this to reflect that most of their recent recruitment had been coming from that County.
[Recruiting in Ireland had been banned following the 1798 rebellion, permitted to restart only in 1806.]
Lots more on the net.
There is a book explicitly on the 35th Regiment.
[Type "35th foot" or "royal sussex" into e.g.
www.webcrawer.com .]
More explicit details can be found in The National Archives at Kew in London:
"The Forlorn Hope"
This book has short histories on each regiment, including general campaign information.
[Available to consult at the Help Desk.]
"History of the British Army"
Huge set of tomes by Sir John FORTESCUE.
[Available to consult in the Library.]
WO 12 records "General Musters & Paylists"
These list the men who served.
Locations and activities can also be seen/deduced.
[Available to consult in the Reading Room.
For 1/35 Battn, from 1808-1821, order WO 12/4960 to 4965
For 2/35 Battn, from 1808-1817, order WO 12/5020 to 5024]
WO 97 records "Royal Hospital Chelsea"
These list men who were pensioned off, if they had completed their service, or had been wounded and declared unfit.
[Available to consult in the Reading Room.
Their names have all been electronically indexed.
Searchable online at
www.nationalarchives.gov.uk .]
[However, Irish men were generally returned to Kilmainham in Dublin.]
Jock