For example, to say 'the cat that eats meat' you say "An cat a itheann feoil". If you said "An cat go n-itheann feoil" you'd actually be saying 'the cat that meat eats', which only makes sense in horror stories.
Had a doubt over 'that is' and checked the prompt. The prompt gave 'go bhfuil'. The answer gave 'atá'.
I have thought about this and surmise that 'atá' is right because it is a relative clause. All the same, the prompt was confusing.