About the 1950's a hybrid chicken was developed that took over the commercial meat market. They had a great feed to meat conversion rate and grew so fast they could be butchered at 6 to 8 weeks and still be larger than many dual purpose chickens at 20 weeks. Since they were butchered so young they were still tender enough to grill or fry. They were developed by selective breeding, no GMO's, hormones, anything like that. The parent breeds used to develop them include Cornish (Indian Game) and Rocks, probably some others. They are generally called Cornish Rock, Cornish Cross, Cornish X, or sometimes just broilers. They are not a breed but are a mix, very carefully bred. They grow so fast that around 8 weeks they start having heart attacks, their skeleton breaks down, or they have other lethal medical problems. It is hard to keep them alive long enough for them to lay eggs. Even if you manage that the males often get so big they can't mate, you would have to use artificial insemination.
Different strains of these have been developed. Two I'm aware of are the Cobb and the one you mentioned, Ross. So your Ross should be the hybrid Cornish Cross broiler. If you are just into meat production you cannot beat the Cornish Cross with any dual purpose bird, their feed to meat conversion and growth rate just can't be beat.
The Indian Game (Cornish) are a dual purpose breed the way Orpington or Sussex are a breed. The Indian Game were used as a parent stock when developing the Cornish Rock broilers because they have a lot of breast meat compared to other breeds, hence the names, but they are totally different critters.
Some people have been successful in keeping Cornish Cross broiler pullets alive long enough to cross them with dual purpose roosters to get a chicken that is a better meat bird than the dual purpose chickens. If you can manage this you can develop your own strain of meat bird. They will still have some of the medical problems caused by fast growth but not as bad as the Cornish X broilers.
There is another type of meat bird, often called Rangers. I don't know what they might be called in Ireland. These are also a hybrid mix but have been developed to grow a little slower than the Cornish X broilers and be really good for pasturing. These can still have the medical problems of the Cornish Cross broilers but not as severe. They could be a lot easier to work with if you are interested in hatching your own. Here is a thread on this forum where someone is playing around with crossing Ranger hens. I think he is doing it mostly for fun instead of seriously trying to develop his own strain but it might be interesting reading.
https://www.backyardchickens.com/threads/crossing-my-red-ranger-hens.1281099/
I'd suggest you get a few Cornish Cross broilers, some Rangers if you can, and a few different dual purpose breeds and raise them. See for yourself how they grow, how you need to manage them, and the differences. It may cost you a year in getting started but I think you will be ahead in the long term.
