The poem's title tells the reader what to expect: a "Requiem" is a song or dirge to commemorate the dead, and "Croppies" refers to Irish rebels in the 1798 uprising against British rule (the name derives from their short haircuts, which were fashioned after those of French revolutionaries).
In the first line of the poem, the speaker immediately identifies themselves as one of those titular "Croppies," mentioning the "pockets of our greatcoats," or long, warm coats. These pockets are "full of barley," a kind of cereal grain that's also symbolic of the Irish spirit of resistance. The mention of this grain hints at the fact that these rebels were predominantly peasants—poor farmers who were by no means trained or equipped to fight.
The Croppies carried grain in their pockets to eat because they had "No kitchens on the run," nor could they stop to set up "camp." The anaphora of the word "no" here emphasizes the Irish rebels' lack of resources, while the asyndeton between these clauses quickens the poem's pace, conveying the rush with which the Irish flee from the British:
No kitchens on the run, no striking camp—
The rebels were forced to "move[] quick and sudden in [their] own country," made outlaws on their own turf. The speaker also points out that "The priest lay behind ditches with the tramp." In other words, the violence affected everyone; farmers, religious clergy, vagabonds—they were all forced to flee, and all united in their desire to be free from British rule.
The sharp /ck/, /ch/, and /t/ consonance in these lines evoke the unpleasant reality of being "on the run," ("pockets," "greatcoats," "kitchens," "striking," "quick," "country," "ditches," etc.). Take a moment to consider the poem's form as well: "Requiem for the Croppies" is a loose sonnet, containing 14 lines of iambic pentameter (a meter in which each line contains five iambs, poetic feet that follow an unstressed-stressed pattern: da-DUM).
On the one hand, this marching meter conveys the swift movement of the Croppies and also connects the poem to a deep literary legacy: sonnets are some of the oldest poetic forms around, and they were famously popularized in English by William Shakespeare.
Given that the heroes of this poem are fighting against the British, however, it's perhaps no wonder that the poet plays with this traditional form. The meter is far from perfect; many of the lines contain more than the expected 10 syllables and swap in trochees (DUM-da) or spondees (DUM-DUM) in place of iambs. Take line 3, for instance:
We moved quick and sudden in our own country.
While overall the poem's rhythm is still recognizably iambic, lines like this one make it feel less formal than a traditional sonnet.
For now, the poem's rhyme scheme seems to match up with that of an English sonnet: lines 1 and 3 and lines 2 and 4 rhyme ("barley"/"country," "camp"/"tramp"), creating an ABAB pattern. Of course, "barley"/country" is really a subtle slant rhyme, as many of the poem's rhymes will be. The imperfection of the verse reflects how untrained and under-resourced the Croppies are compared to the British troops.