OK, then a few pointers -
As you will be working for companies, I would assume that their insurance will cover you, and you don't need any indemnity cover - not usually necessary unless you are giving "advice" that you could be sued for. Therefore Incorporation is not necessarily the best choice doe to the increased costs and administration of dealing with a Ltd company. But - there are exceptions to this and you should really get professional advice in this regard.
As long as you are working ad-hoc for several companies you should not be considered to be an employee - is this a possibility? Otherwise, the Revenue might consider you an employee anyway - but the risk for this is with the companies (generally) and not you.
Usual expenses for this type of self- employment (not Incorporation):
Use of home as office - having part of a room (not a whole room, which could give rise to CGT on sale of premises), claim running costs as a % of the whole house - mortgage interest, heating, lighting (will be minimal I would assume), but you need to keep full records. This can be a pain, and in some cases not worth doing - worth considering though.
Use of computer - depreciation fo same, and costs, paper, ink, etc for bookkeeping and invoices as a % of total use. Revenue are unlikely to accept 100% business use.
Phone - better to have a separate line or mobile for this due to dual purpose potential of home phone line.
Travel - as a self-employed person, use of car has to be calculated on overall basis - depreciation of cost, petrol, insurance, NCT, repairs, petrol- claiming % used for business, and you will need to keep a log of total and business mileage for Revenue - rates per mile as laid down by Revenue may only be used by employees.
Accountancy fees
Off the top of my head I can't think of any other costs you could incur, but you will know best if there are any other expenses to be considered.
If you are intent on doing all of this yourself, I would suggest you take professional advice for the first year, and then carry on yourself in the same vein. However, if there would be any change in circumstances, income, costs etc, it would be best to get full advice.
Hope this helps