"Jahre" is the plural form, meaning "years". The singular is "Jahr", year. By the way, remember to ALWAYS capitalize nouns in German. :-)
Cylinders.
McDonald's
boats
Martinez's
cobwebs
You wouldn't pluralize "processing", but you would pluralize "process" making more than one process "processes".
Handkerchiefs is correct.
Fournesses is the plural of Fourness
Sirs is the plural of sir
Folisis is the plural of Folisi
Henrik Jahre was born in 1937.
Jørgen Jahre died in 1998.