Worth Doing
noun
The やり part is from やる which is "to do." So, if the effect (甲斐) is good to do, then it is worth doing.
やりがい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
この仕事は、誰かのために役に立つことができるので、とてもやり甲斐があります。
This is a satisfying job for me because I can be useful to other people.