WordNet sense 1 (not easily irritated):

equable, even-tempered, good-tempered

WordNet sense 2 (having an easygoing and cheerful disposition):

good-natured

WordNet sense 3 (living without undue worry; taking life easy):

easygoing

WordNet sense 4 (satisfied or showing satisfaction with things as they are):

content, contented, content

WordNet sense 5 ((of a body of water) free from disturbance by heavy waves):

quiet, smooth, still, tranquil, unruffled

WordNet sense 6 ((of weather) free from storm or wind):

calm