See [broken link removed] for details on exemptions that apply to certain agricultural structures on farms. So you can go larger than 40sqm, but the usage is restricted.
What do you mean by mobile business? If it's not directly related to agriculture, then you will most likely need planning permission for a change of usage.
Planning rights can change over time as new legislation passes or a new local development plan comes into effect.
Farmers have the right to extract water from streams running through their land, but they must consider the rights of other landowners downstream in the quantities they extract. If it's a larger river classed as a navigation under the remit of Waterways Ireland, you will need their permission to extract water.