"I do not like that."
Translation:Ní maith liom é sin.
The positive form of this statement is Is maith liom é sin.
Níl is only ever used when you are negatimg a tá statement, so níl is not used when you are negating the copua is, and níl is not used when you are negating any other present tense verb except tá.
ní ithim cáis - "I don't eat cheese"
ní ritheann sé - "he doesn't run"
ní cheannaíonn sé bainne - "he doesn't buy milk"
níl sé deacair - "it isn't difficult" (negative of tá sé deacair)