There is no carvery scene in Cahir. Sunday lunch here is a hotel dining room, a restaurant running its normal menu, or a pub kitchen doing whatever it does on a Sunday. Come off the M8 expecting a row of carveries and you will be disappointed. Come for one good lunch and you will be fine.
The hotel
The Butler Room at Cahir House Hotel on the Square is the main Sunday sitting in town. The room is named for the Butler family, who owned the castle across the road and lived in this building in the 1770s, and it is the most formal dining room Cahir has. Worth knowing before you book: reviewers who arrive expecting a traditional carvery sometimes find an a la carte menu instead, so if a plated roast is specifically what you are after, ring and ask what is on that Sunday.
The restaurants
Lava Rock on Castle Street serves Sunday lunch and is the restaurant most people in Cahir would name first for any meal. It is modern international and grill rather than a roast specialist, it seats about fifty, and it operates bring your own wine.
The town's Italians, Genoa and Galileo, are the other answer to the same question if nobody in the group is set on a roast.
The pubs
The Galtee Inn on the Square is the most reliable of the pub kitchens and the closest thing in Cahir to a plain, generous Sunday plate. The Shamrock Lounge and The Hill Inn on Barrack Street also do food, and the Hill Inn has a beer garden and a function room, which makes it the easier one for a large family group. Pubs in Cahir has the full list.
If you will drive
The Old Convent at Clogheen, about fifteen minutes south on the Vee road, is the destination cooking in this part of the county. It is not a Sunday roast in the pub sense and it needs booking well ahead, but if the point of the day is the meal rather than the roast, that is where to go.
Practical
Sunday is the one day a small town kitchen is either flat out or shut, and which of those you get is not predictable from a website. Ring. A table booked on Saturday costs nothing and saves the drive.
The rest of the eating in the town is on the best places to eat in Cahir, with cafes in Cahir for daytime coffee and takeaway food in Cahir for the evening if the lunch ran long.