The poem uses imagery to bring the 1798 Irish uprising against the British to vivid life and also to juxtapose the rebels against the British soldiers.
The poem begins with a bit of imagery that immediately indicates that the speaker is one of the "Croppies"—Irish rebels named for their short haircuts:
The pockets of our greatcoats full of barley—
The "Croppies" were known for keeping "barley" in their "pockets" to eat because they couldn't afford to stop and cook. The specificity of "greatcoats" (a kind of long, heavy overcoat) and pockets stuffed with "barley" (rather than grain more generally) evokes a particular time and place and suggests the desperate situation the Irish rebels were in: they've shoved raw grain in their pockets.
In lines 7-9, the speaker goes into detail about the various "tactics" the Irish used to outsmart the British troops. They "cut through reins and rider" (i.e., soldiers on horseback) "with the pike," a rudimentary weapon made of a wooden shaft with a pointed steel blade at the top. They drove cows into the ranks of trained, armed soldiers to break them up, and they "retreat[ed] through hedges" where horses couldn't go.
This imagery illustrates the cleverness of the Irish while also revealing how woefully disadvantaged they were. Their lack of weapons and training ultimately results in "Terraced thousands d[ying], shaking scythes at cannon." In other words, lined up shoulder to shoulder and armed only with farming tools and antiquated weapons, the Irish stood no chance against the surrounding British with their arms and ammunition.
The poem then personifies the hillside in line 12:
The hillside blushed, soaked in our broken wave.
On one level, this is simply another striking bit of imagery: the British slaughtered the rebels so thoroughly and brutally that the landscape itself was "soaked" with blood. The metaphor of the fallen Croppies as a "broken wave" adds to the intensity of the scene, conveying a rush of blood crashing into the earth upon their defeat.
The personification of "hillside" also suggests the shamefulness of what the British did: annihilating thousands of people who were inadequately armed and trained. It's as though the hillside itself is embarrassed or shamed by such brutality.
The speaker goes on to say that the British "buried [the Croppies] without shroud or coffin," meaning that they just threw their bodies into one large, unmarked grave. Finally, the speaker says that "in August the barley grew up out of the grave." This imagery suggests that the Irish resistance couldn't be quelled, not even by all this crushing defeat. The seeds for future revolutions had been sown with the Croppies' sacrifice.