While 'gombeen man' does indeed mean usurer, its meaning has changed a bit over time; it is not at all uncommon for modern writers to define it as "a money lender at the time of the Irish potato famine", and this is the general association it tends to have today.
The gombeen man predates the Irish potato famine, although there wasn't a lot of interest among the English in scrutinizing Irish ways of life before this, except as pertaining to religion and rioting, so we don't have much documentation of the details. Writings from the 1880s tend to refer to gombeen men as moneylenders changing interest rates of 20 to 50% interest, although sometimes higher rates are referenced. Bernard Henry Becker recorded in Disturbed Ireland (1881) the common practice of leading small sums with the understanding that the debtor would pay one shilling per pound per week as security until the original debt was paid back -- the equivalent of a 260% interest rate, if calculated per annum. Oddly, he considered this charge per week to be the mark of a gombeen man, while the more modest rate of one shilling per month (coming to 60% a year) was a more respectable brand of usurer; it should be noted that English law at this time capped interest rates on loans at 5%.
Circa 1830, the potato was quickly becoming the staple crop in Ireland -- and not the way wheat is a staple food in America; more like how oxygen is a staple gas. The growing population and expensive rents on agricultural land meant that the majority of the population was falling into a trap in which they fed their family on the nutritionally complete and easily produced diet of potatoes and milk, while using the majority of the rented land to raise crops to be sold in England. Unfortunately, potatoes do not store well year-to-year, and you would not dare to eat the crops that were to pay the rent, as being homeless in Ireland at this time was not much above a death sentence. Thus, the months of June, July, and August were known as the 'meal months' -- those months during which a family might find themselves short on potatoes, and be forced to buy meal to eat. Enter another form of gombeen man.
Unscrupulous shopkeepers and money-lenders had a trick up their sleeve: sell needful goods on credit and charge interest on the debt, or alternatively, keep things simple and don't charge interest -- just sell on credit at exorbitant prices. These were, naturally, high-risk high-return sales, and were offered by the comparatively wealthy to the very poor; some called these operators gombeen men, while others preferred to limit the term to money lenders. While I have no strong evidence, I suspect that the Irish were not as picky as the English when applying this term; there are a lot of Englishmen writing about what they think a gombeen man is, but the illiterate Irish masses are not well represented.
Regardless, the gombeen man took a hit to their already tarnished reputation during the great famine, when hundreds of thousands of families had to chose between starving now, or eating and losing their land only to starve later. Since that time the term has been largely a term of scorn and carries the implication of amoral and shady dealings; not admittedly, a big change.