The imperative would be used, and its conjugation would depend upon whether the second-person singular or second-person plural was intended. I don’t know what the translation of “fire walk” actually is, but my guesses would be Síúil liom ar tine for the singular and Síúlaigí liom ar tine for the plural.
I think the main difference is that "need" isn't as strongly binding as "must". I don't know enough Irish to say what that aspect of it is (or if you can use "must" as a logical conclusion [e.g. "They must have done it."]), but the English use differs by a few key elements.
1) "I need to pee." => You wouldn't use "must" here, because "need" is being used as a stronger form of "want". You can say "I must pee", but only if you are already on your way to the bathroom (and if you are British).
2) "I want (to have) them." vs. "I need (to have) them." vs. "I must have them." => The difference here is degree. "I want" is relatively weak, and can indicate a passing craving or an idle desire. "I need" is stronger and indicates something that can not be done without (e.g. "I need food to survive."), or something that is necessary in the long run, but not necessarily at the moment (e.g. "I need $20 by Thursday."). "I must have" indicates an immediate need for something (e.g. "I must have food! I haven't eaten anything in weeks."), an all-encompassing need for something (i.e. Any movie villain that absolutely needs a specific item), or an obligation to do or have something (e.g. "I must fight that monster, for no one else can."). Alternatively, "must" can be used as a logical concluder (e.g. "James must have let the dogs out; he was the only one home."), except occasionally in [future possibility tense (I'm blanking on the actual name, but it's not quite when something might be; it's future tense, but not the main verb, it's as part of a phrase with a question word. Just ignore this ramble if it doesn't make sense.)] (e.g. "I don't see why it has to be me." vs. "Why must it be me?").
3) There are various other special cases where "must" and "need" aren't interchangeable, but I cannot think of any at the moment.
Hmm... I just found this via a different discussion. I'm afraid I disagree with it. The particular case here is whether (someone) needs to (do something) and (someone) must (do something) are equivalent, and as another poster has said as well, in my (native Australian) version of English, they are.
We tell the children at my work: "You must wear a hat" and "you need to wear a hat". No difference in meaning. (Have you ever seen videos of police telling people "you need to calm down"?)
For your point (2), I will often say "I need food", meaning I am hungry and I "must have" food right now before I collapse.
(3) The only other case I can think of is the British Highway Code, which uses "must" and "must not" for particular legal requirements.
The key point is that Tá ar X Y and Tá X ó Y mean very different things in Irish - translating them both with "need" is both confusing and unnecessary.
I would also point out that "I need to X" carries a hidden implication of negative consequences if I don't ("you need to calm down, or else", "you need to wear a hat, or you'll catch a cold", "I need to eat, or I'll collapse") whereas "I must X" is more about obligation than consequences, and doesn't have quite the same connotation. Most of the time, they're the same, but not always.