It is no longer possible for developers to provide a cash contribution instead of housing units. This follows from a high court case with Rathdown DunLaoghaire last year.
Previous to that, a lot of councils requested cash payments rather than housing units. It really was up to individual councils in negotiation with each developer.
They must now be given houses, the number is determined under the Part V rules (c. 20% of the difference in value of the land before planning and after planning)
There may however be some developments where agreements were made before the above court case but have not yet been completed so they may be giving cash payments instead.