"Collect wild crab apples after first frost," writes Mary Ward of Nenagh, County Tipperary, who likes to serve this jelly with cold roast lamb. "Check that the pips [seeds] are brown." If you've got a crabapple tree or two in the backyard (and it gets frosty in your neighborhood), you can follow her advice. Otherwise, look for the fruit at farmers' markets.
Makes enough to fill 6 to 8 one-pint/475-ml jars
10 lb/5 kg crabapples, cored and coarsely chopped
1 cup/150 g blackberries
6 whole cloves
sugar as needed
Put the crabapples and blackberries into a large pot. Add the cloves and cover with enough water to come within 1 in/2.5 cm of the top of the fruit. Bring to a boil over medium-high heat, then reduce the heat to medium and allow to boil, stirring frequently to keep the fruit from sticking to the bottom of the pot, for about 1 hour.
Choose a large bowl that will fit inside the pot with the crabapples. Set 8 layers of 20-in/50-cm square sheets of cheesecloth over the bowl letting the cheesecloth hang amply over the sides. Spoon the apple mixture and pot juices into the middle of the cheesecloth; then rinse out the cooking pot. Set the bowl down into the clean pot. Gather the opposing ends of the cheesecloth and tie securely to make a sack. Suspend the sack from a rolling pin or broom handle set across the edges of the pot and allow the fruit to drain into the bowl overnight.
Measure the juice that has drained into the bowl; then put into a medium saucepan with an equal amount of sugar. (Discard the solids.) Bring to a rolling boil over high heat. Boil until the temperature reaches 220°F/100°C on a candy thermometer, about 15 minutes, skimming any foam that rises to the surface. Reduce the heat and test to see if the jelly has set by spooning a bit onto a chilled saucer. Allow the jelly to cool slightly, then tilt the saucer to one side. If the jelly remains in a blob with a few ripples, it's ready.
Divide the jelly evenly between 6 to 8 sterilized 1-pint/475-ml jars, then seal with sterilized rings and lids. Transfer the filled jars to a canning rack, submerge in a pot of gently boiling water (make sure the jars are covered by at least 1 in/2.5 cm of water), and boil for 5 minutes. Carefully lift the jars from the water with jar tongs and place on a dish towel to cool, undisturbed, for 24 hours.