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.
I am amazed at this confusion between need and must coming from native english speakers. In english, to the best of my knowledge and confirmed by a quick dictionary check, "must" express an obligation (physical, legal or moral, although moral obligation is best expressed with "have to"), while "need" express simply necessity, and those are quite different concepts. Pple who say "i need to pee" and "i must pee" are equivalent simply speak vague english, although they might see that if they try and imagine a case where they could say "i need to pee, but i must not pee", or "i must pee, but i need not pee". Quite different predicaments, aren't they?
Now apparently, the basic meanings of must and need are expressed by two different structures in Irish, that is well, good and simple. A more relevant question would be if their secondary meanings are also expressed in the same way, like the uses of "must" to express logical consequence ("if A is true, then B must (surely) be true") or supputation ("i don't see him, he must have left"): would those be expressed in Irish with the "Tá ar" structure too?
In the case of supposition ("he must have left"), you can say "caithfidh sé gur imigh sé" (it must be that he left) or "ní foláir nó gur imigh sé".
In the case of "If A is true, then B must be true", I'm not sure you'd say that in Irish - you would say "If A is true, B is true" - "má tá A fíor/ceart, tá B fíor/ceart". If it isn't a strictly logical consequence, but just an extremely likely consequence, then "caithfaidh sé go bhfuil B fíor/ceart" would probably be better.
I don't believe that you would use "tá ar" in either of those cases.
Isn't "siúl" used to mean "go" as well, in the generalized sense (e.g., I have to go now)? I typed "you must go" and got it wrong, so I was just wondering.
Made me think of the macaronic song "Siúil A Rún" where she's giving him permission to go fight even though she loves and fears for him. She's saying "go, my love," not "walk, my love." Can anyone edify?
No, as a straight verb like this, siúl means walk, but siúl is used in phrases that imply motion, and that sometimes use "go" in English, such as "cad atá ar siúl?" - "what's going on?".
In "siúl, a rúin" she's not telling him to go so much as giving him her permission to go, and it could be interpreted as "move on" or "travel if you must", or even "walk away".