Given the amount of work that needs to be done to make sure the border between NI and the Republic operates properly, I really can't see too much of this money that wouldn't have been spent there anyway. It's the most troublesome issue there is. As long as Trade is all sorted, the rest of the issues ought to resolve themselves. But the land border between two fractious nations and the potential for damage if either side get this wrong means that it's gonna need money. And I'm pretty sure some, if not all, of this payment is ultimately going to pay towards that. I doubt it's ringfenced for anything else, because that would be public by now.
I would also need to point out that the amount of cash that was identified by Corbyn as coming from this 'magic money tree' was far, far in excess of £1b. Tuition fees alone would have been many times that amount.