According to B.Quayle (General View ..1794):
The lower class of inhabitants live on meal of oats, and barley, and potatoes, and fish, with a small proportion of fleshmeat. Their breakfast is of meal pottage and milk; their dinner is potatoes, and either salt or fresh fish; sometimes beef or pork, as they can afford; and their supper either pottage, or potatoes and milk. What bread they use is made of barley meal, which is cheaper than oat meal, and is considered as more wholesome. The labourers have a small piece of ground for the growth of potatoes; and those who keep cows are obliged to cultivate some corn for their provender.
He also goes on to say:
Almost every cottager keeps one or two pigs, which makes their number considerable; they are reared on the offal of the houses, and run about the lanes, and are killed at ten or twelve months old. Potatoes and grains assist their keep in summer, and potatoes, whether boiled or raw, with some little corn, is the food used for fatting. Hams and bacon for the English market prove a great encouragement for this species of stock. The market price for fresh pork is two pence-halfpenny per pound, and for hams and bacon five pence. The average weight is fifty pounds per quarter. Poultry is also numerous, not being annoyed by foxes, foumerts, or other vermin. Fowls fetch at market six-pence a piece, and weigh four pounds at an average. Geese are very common, but are considered disagreeable companions to cattle in pastures: where they can be conveniently reared, they are well attended to; their price is from one shilling to eighteen pence, and weigh fix or eight pounds. Turkeys and ducks are also plenty; a considerable quantity of poultry and eggs are sent to England.
Roeder gives an account given to in in the 1880's by old Manx people of food and drink presumeably dating from the late 18th century confirming that milk, oats and potatoes formed the major part of the diet.
Clague also gives an account of older Manx traditions in food