What is the conflict of this ballad, if you analyse it as a work of fiction/short story?
It seems like the conflict is the nobles having to go out to sea (knowing that it would mean that they will die) because they have a duty to their king. It's hard to tell, because I'm not sure if it is Sir Spence stating "Mak hast, mak haste, my mirry men all, Our guide schip sails the morne," or if it's the king saying to both Sir Spens and the men, and if it's the men stating "O say na sae, my master deir, For I feir a deadlie storme" to Sir Spence or the king.
In "Sir Patrick Spens," Spens does say the words you attribute to him, not the king. He exhorts his men to set sail. The men also make the reply that you cite. They realize the danger of being on the sea in this season.
The element of Spens and his crew having to fulfill their king's wishes is present. They know they are doomed but they must answer the call of royalty. This puts them into a no-win situation. Situational irony is present, with Spens and his crew trapped in a situation that is beyond their control.
Implied, however, is the stronger conflict between the elderly adviser to the king and Spens. The adviser presumably also knows the danger, and recommends Spens as a way of getting rid of him. He executes, in a sense, a legal murder. Though the reader gets no information on what the conflict is between the adviser and Spens, it is the igniting conflict of the narrative. It all comes back to the use of power. The king has the power, and the adviser has the king's ear. And Spens can do nothing but play the role of pawn.
One problem is that this ballad belongs to an oral tradition, and though the best known version is the one collected by Child (58), its having been transcribed in the nineteenth century by Francis James Child does not make it somehow the most authoritative version. Instead, many different variants of the song were performed, some with the wreck occurring on the outward voyage and some with the ship being wrecked on its return.
The key elements of the narrative are that Patrick Spens, a man renowned for his skill as a sailor, and his men undertake an urgent mission to Norway at the behest of the King despite bad sailing weather and undergo a shipwreck.
The key conflict is that between human skill and nature. One common part of scene-setting in oral traditional works is some form of boasting in which praise is heaped on the hero and vituperation on the opponent. Thus we get praise of Patrick's sailing skill (when the adviser recommends Patrick to the King) and the comment about the phase of the moon, which amplifies the dangers of the voyage. In the Child version of the ballad, the description of the sinking of the ship is quite brief, but in other variants, it is far more extended.
The king informs Patrick of the need to set sail by a letter; the two are never physically in the same location. "Mak hast..." is spoken by Patrick to his men. "O say na sae..." is spoken by one of the men to Patrick.