I'm not sure why it couldn't be "I have interest in...". It's still indefinite, and it's something I would actually say.
The Irish "sa stair" literally means "in the history", so I think it should accept "I'm interested in the history."
It seems to me that Irish treats the definite article more similarly to Romance languages than English. In both, one studies the maths and the history, and one travels to the Italy and to the Greece. This translation sounds more right to me than including the in the English.
To answer my own question nine months later: because stair is an abstract noun, and it’s being used in a general sense in the sentence above, it takes an article in Irish. The article being needed in Irish does not imply that an article is also needed in English.
is there a difference between "spéis" and "suim" or are they completely interchangeable?