"Gombeen" is a common Irish insult. It is related to the phrase "gombeen man", but does not necessarily have the same meaning. To call somebody a gombeen normally indicates that you consider his or her mental capacity subpar. It can also be used to indicate somebody of a lowly nature who is beneath estimation. However, it can also be used to associate somebody with the practices of the gombeen man, i.e. petty usury and exploitation. Thus, to call somebody a "gombeen businessman" is to accuse him of using petty, exploitative practices in his business.
All in all, a very adaptable insult. It can also be shortened to "gom", for when you're in a particular hurry to cause offence.