Probably there is an “é” that is omitted like “Is maith liom é”. The maith and the é are what is “coupled”.
Yes, there is an “é” that is sometimes omitted. I think it is grammatically incorrect to ommit it but it still is seen that way I guess for short. I always write it “Is maith liom é”. It’s true that this one and other idioms like it do not follow that rule, but as a whole those exceptions are few and are easy to learn and memorize. 🙂