I guess it is a matter of return in investment. A 29k railcar costs roughly 2 million euro, so to use half of a car for a snack unit requires a million euro investment and would require an extra staff member to operate unless they stopped the trolley which is always going to make more money.
When they had a buffet car on the old Sligo intercity all they really sold was drink and sambos which can be sold off the trolley just as easily.
What is peculiar about the Sligo line is that the majority of its traffic is between Longford and Dublin which is just over 90 minutes. Relatively few of those passengers are going to be interested in a hot meal and certainly not at the prices Irish Rail charge for food - and would be necessary to recoup the cost of staffing and investment.