I'm fresh from making my first tax return under self-employment. I found the Revenue guide "IT48 Starting in Business" very useful... it doesn't seem to be available on the Revenue website but I found it on the Enterprise Ireland website. I can't post a link because I'm a new user but if you google "IT48 starting in business" you'll find it.
Page 11 of IT48 says this about "Pre-Trading Expenses"
"What about pre-trading expenses?
A business, whether incorporated or not, can claim for certain
pre-trading expenses when calculating the trading income. A
deduction is available for pre-trading expenses which:
-Are incurred in the three years prior to commencement of the trade or profession,
-Would not normally be allowable.
Examples of pre-trading expenses are:
-Accountancy fees,
- Advertising costs,
- Costs of feasibility studies,
- Costs of preparing business plans,
- Rent paid for the premises from which the business operates.
The allowable amounts are treated as having been incurred at the time the business commences. Allowable amounts cannot be set off against income other than income from that business but can be carried forward and set against future profits of the business."
When you make your tax return you can have more than one trade, but I don't know if you have to register each trade separately.