You and your siblings, each, are eligible to inherit 400000 in total from your parents so any previous inheritance (if any) from your father must also be accounted for( or any other group A inheritance).
Revenue link
here
What is the total value of your mother's estate? (House/land/money/bank accounts etc).
This is divided by 4.
If this amount (including any previous inheritance (s)) is less than 400000 then you have no inheritance tax liability.
Regarding the land.
You indicate it's a series of separate fields.
It could be possible to get some local auctioneer to value each isolated plot.
Then you may be able to take 1 or more fields equal to your share.
Now the fun starts.
It is possible to disclaim an inheritance.
However, I do not know if you can disclaim parts of must disclaim it all.
Also whether you can specify who is to get it or if it just remains part of the estate I don't know either.
Should you take your share and then gift your share of the house to your brother there may be a tax liability for
him not you.
The group B threshold would apply, this would have to include any "gifts" from his other siblings.
There may be a tax liability but it's most likely your brothers not yours.
If you disclaim the inheritance on the understanding that you will get a "site" later is probably another minefield.
I think you need proper legal advice before agree to anything.