There are a number of issues here so I'll try go through them in turn.
1) The €35,000 you paid for the land is not an expense. That is a capital asset. You do not get a deduction for the €35,000 against your tax. If you ever sold the land, then any amount over €35,000 would be liable to Capital Gains Tax ( subject to agricultural reliefs which may reduce any CGT. ) If you had borrowed to purchase it then the interest on the loan would be allowable as a deduction for tax.
2) The area aid payments are income and must be included with your return. They would be part of overall sales e.g.
Sale of Cattle
Area Aid
Sales of crops
The area aid payments are from government agencies so must be shown in that part of the accounts schedule on the tax return.
3) Assuming that you are jointly assessed for income tax then any loss in one trade, e.g. yours is allowable against other income in the year ( including his trade income ) or can be carried forward to set against future profits from his business. There is a specific entry on the tax return to indicate if you want to set the loss against other income.
4) The slatted unit. That is allowable for tax but not all at once, but 12.5% pa over 8 years. Unless it was through the Scheme for investment in Farmyard Pollution in which case it is allowable at 33.33% over 3 years. The reason for the longer write off is because it is not a straight expense like, feed or ferts but is something which lasts over time. You claim that in the Capital Allowances section of the tax return.
I appreciate that your farm and his business may be small but given the several sources of income and losses it would be well worth engaging the services of a good local accountant to do up the tax return and accounts for you. The costs are allowable against tax and may well save you more than you think.
I hope this clears some of the issues for you.