that hunger Sent us out with milk cans, pea tins, jam-pots (lines 8-9)
Holy containers! If someone told you money was growing on trees, you'd grab anything that would hold the treasure too – garbage bags, laundry sacks, and so on; These boys are out to get as much as they possibly can.
We trekked and picked until the cans were full, (line 12)
Their greed is tireless. They won't stop until the moment they can't hold anymore. If they were stronger, you get the sense they'd carry boatloads of berries.
We hoarded the fresh berries in the byre. (line 17)
The verb "hoarded" gives away the greed theme big time. Heaney easily could have said "stored" but this is a type of storage that's specific to greed. The byre also gives a sense of secrecy; that they're keeping their berries away from everyone else. No sharing going on there!
glutting on our cache. (line 19)
"Cache" means something that's hidden or inaccessible to anyone else. Those boys meant to keep the berries private; their own secret treasure trove.