Not sure how much digs are but I imagine it'd depend on whether you were including food in price. The colleges have websites which indicate how much students should expect to pay.
I've also heard that people charge over holiday periods. I suppose it guarantees that the room will still be available. Otherwise students might have to move all their things out and hope you hadn't let the room to someone else while they were on holiday.
Depending on what you are charging, you may be able to avail of the 'Rent a Room Scheme'. This means that you can earn 7,600 per annum tax free. You do have to register with the Rev. for this scheme. If this is the case, then you don't have to register with the PRTB.
I don't think you need to concern yourself with rent allowance but there are other posts about this, so maybe you should do a search.
Hope this helps.