Slane is at a club where they have just employed a new manager who has a big reputation for giving youth a chance. He is now in the position where he has a good season or two ahead of him where he can expect to play at least semi-regularly for the our first team in the SPL.
Only a total mongo would choose this as the time to move to Celtic and languish in their 19s / reserves for, realistically, another couple of seasons before the chance of first team football comes round again.
IMO There is no way he will leave, and based on his recent performances and visible potential, there's no way MFC would accept any kind of derisory offer, and there is no way Gammon will allow his contract to peter out to nothing