Before the second line is over, the universe has ended. Stars dissolve, light sinks into "ne'er ending chaos and night", and only then does the speaker reach his real subject: the courage of Erin, which "may never decay". Shelley writes in the voice of an Irishman, as the title announces, and the poem's central idea is that a people's courage can outlast everything, even defeat and death. What follows is a catalogue of exactly the devastation the opening stanza claimed to rise above. The poem keeps asking whether courage can survive ruin, and answers by turning it into a demand for vengeance.
The first stanza sets the pattern. The lines gallop in anapaests, with a rolling beat like "the STARS may dis-SOLVE, and the FOUN-tain of LIGHT", and they pair off in rhyming couplets that arrive like drumbeats. The couplets also do some quiet work. "Decay" is the word the speaker denies, yet it is bound by rhyme to "vanish away", so the thing he refuses to admit sounds its echo in his own line. His boast is more anxious than it first appears.
In the second stanza the cosmic gives way to the visible. "See!" he says, and the poem turns into a landscape of wreckage. The hypothetical "our mansions must fall" has already become fact: "Our ancestors' dwellings lie sunk on the ground." The verbs are heavy and horizontal (sunk, lie, stretched), and everything proud has been laid flat. Meanwhile the enemy is upright and mounted: "Our foes ride in triumph throughout our domains." The word "domains" makes the conquest feel like a theft of property as well as a military loss.
The third stanza moves from sight to sound, and here the poem's thinking is most interesting. The harp, a traditional emblem of Irish culture, is "dead", and its "rapturous measure" has sunk. "Measure" can mean music or metre, so the speaker is mourning the very kind of verse he is writing. Its place is taken by "the war note", "the clangour of spears" and "the dread yell of Sloghan", a battle cry (the word we now spell "slogan"). Song has been replaced by noise, and the poem's own insistent rhythm starts to sound less like a ballad and more like a march.
That noise grows louder in the final stanza, which opens with the old lament of "where are the heroes?" The answer is brutal and contradictory. They are "triumphant in death" yet "convulsed", lying on a "blood sprinkled heath". The phrase "triumphant in death" is a stretch, an attempt to dress defeat as victory, and the jarring word "convulsed" undercuts it by showing the bodily pain. Then the dead refuse to stay dead. Their ghosts "ride on the blast that sweeps by", a deliberate mirror of the enemy riders in stanza two. The conquerors ride the land, and the slain ride the wind. The poem ends on their cry, "my countrymen! vengeance!", set in quotation marks so the dead speak for themselves. The adverb "incessantly" stretches the last line out, so the poem closes with no peace and no victory, only an unanswered demand.
The poem's significance lies partly in how conventional it is. Its diction ("wont", "ne'er", "Ah!"), its exclamations and its stock imagery of heaths, harps and heroes belong to the patriotic lament, and Shelley handles them with energy rather than subtlety. Still, it is revealing to set it beside Shelley's "Ozymandias", where a ruin is also a lesson. There the ruin is viewed with ironic distance, and the fallen thing is a tyrant's vanity. Here the ruin belongs to the victims, and instead of irony it produces anger. The poem treats devastation as a call to action, and its chief claim is that courage, once the buildings, music and bodies are gone, survives as a duty the dead hand on to the living.