Its origin is the Irish language word "gaimbín", meaning monetary interest or usury.
While the phrase "gombeen man" is traditionally intended to refer to an unscrupulous and petty-tyrannical Irish Catholic who exploits his own people, it can be applied in relation to other groups such as, in this instance from James Joyce, to an Irish Jewish man: -- O, Father Cowley said.
Crime writer Kyril Bonfiglioli wrote a dark short story called "The Gombeen Man" about just such a character in the late '70s.
This excerpt is from The Crock of Gold, by James Stephens: "... the women were true to their own doctrines and refused to part with information to any persons saving only those of high rank, such as policemen, gombeen men, and district and county councillors; but even to these they charged high prices for their information, and a bonus on any gains which accrued through the following of their advices [sic]."
In Irish politics, it is used to condemn an opponent for dishonesty or corruption, although its definition has become less precise with time and usage and it can also imply pettiness and close-mindedness.