From what I've heard, a teacher wouldn't usually let you continue with HL if you didn't get at least a C in HL for the Junior Cert. Theoretically it might be possible, but even if you were a
allowed to continue with HL for the LC, I'd be more inclined to drop down or pick another subject if possible, since failing the subject at JC level isn't a good sign for the LC. If it's a theory subject, something not too difficult to understand, you mightn't have that much trouble improving your grade if you're prepared to study hard, but for something like Maths or Irish, I would recommend dropping down. Do you need that particular subject at HL level for a course you're looking at?
It depends on the teacher to be honest, some might let you off if they think you're able for HL, but some mightn't. There's no set rule for it that I know of, so if you do fail you'll just have to try and talk your teacher into letting you do HL. What subject(s) do you think you might have failed?