Having grown up in Reformed circles, I would say @lakshmi hit the nail on the head. 
‘Irresistible Grace’ is a concept that flows logically from other founding concepts in the Reformed story of Christ’s redemption of humankind…namely, how it is that the undeserving are saved through absolutely no merit of their own…how the whole process of salvation is a work initiated by God.
I often heard it argued that if we had the ability to choose God, then our choice of Him would technically be a ‘work’…and, thus, we would be saved via a work. But, of course, as Paul argues, in Eph. 2:8-9…
For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast.
I’ve also heard Jesus’ words in John gospel used to illustrate the concept of grace being 'irresistible…or, at least, that the call to faith, which is initiated by the Holy Spirit, is ‘effectual’:
No one can come to me unless the Father who sent me draws him. [6:44]
I love that you’re reading about the Reformation, @alison, for the philosophy/theology that underpins our story re. the process of salvation is super important…esp. when it regards the bondage of our human will!