Are we not missing a few fundamentals here:
Market Value 5/4/74
Indexation
Current Market Value
etc
to see if there is even a gain!
Would retirement relief apply here?
We sure are missing them, but since the OP hasn't deigned to give them to us we can only answer on the basis of the facts he's given!
He specifically asked about the consequences of his giving a "gift" (which would actually be viewed as consideration on an objective view of the transaction) in return for his parents transferring the land.
The answer to that is there is no consequence to that gift, as market value will be imposed on the transaction.
Any reliefs that can apply, will apply to that market value, regardless of the gift from the OP.
I'd suggest that it would be very unusual that there wouldn't be a gain from the parents' side, if it's been held for any substantial period of time - agri land prices have bounced back since 2010 AFAIK, and the average price achieved per acre in the last 2 years was ~€10k.
Agri land average price in 1974 was around £600 - so say €800 per acre. The max indexation factor is 7.528, which gives a base cost of €6k - €7k at best. Anyone arguing a higher base cost than this ballpark would have to able to make a fairly compelling case.
The OP had a duplicate thread (since deleted) where he indicated he believed retirement relief would apply, so I think the whole question is a moot point anyway.