Hotel employment: is this legal?

My sister got a job in a hotel in Ireland. She was told in the interview that she would get 39 hrs a week at €10 per hour. They also mentioned she might once in a while have to do an overnight stay. She got her contract which says the same. Then she got her time and noted she was in for an overnight once a week. This entails sleeping in the hotel from 11.30pm-6.30am. She is under NO circumstances allowed to leave the premises and along with the one nightporter is to be the responsible staff member in the event of a fire.

She received her first paycheque and noted she hadn't been paid for the overnight. She went to her manager and asked her about it and was told no she would never be paid. She replied "Well it must be voluntary then, in which case I'll opt out". But the manager said no it wasn't an option and she should speak to the director/owner.

She went to speak to him and the director said that in fact the €10 an hour amounted to €390 per week and covered the 46 hours (39 + 7). My sis responded that no that was below minimum wage as it calculated to €8.47 per hour. The director then replied that in fact he forgot to add in the daily food allowance of €4.60 per day which when taken over 5days of work and added to her salary resulted in €8.97 per hour.

She didn't pursue it any further, but a few days later was pulled aside by her manager to ask her how all was going. She said that she was very unhappy because she felt she'd been decieved by how often the overnight would be, and about her wage given that her contract STATES that she will be paid €10 per hour and not €8.97 per hour. And in no place does it state that the overnight is unpaid. Thus, she finds the company in breach of contract.

Her manager told her that she understood and were she required to do the same she too would be unhappy. She stated however that she knows the hotel has run this system for years and so must be within the law and if she is unhappy she may leave!

The thing we are all finding so shocking about this is her Co-workers. Some of them have been doing this for 11 and 13 years, they have small children and have had at times to do two nights per week. One asked not to have to do it anymore and was reduced to parttime hours and still given one overnight a week. Two members of staff are parttime and are sometimes required to do two unpaid overnights, essentially halving their salaries!!!??

I have looked at this from every angle, and I spoke to NERA. They said if she has to remain at her employer's behest then she must be paid. Similarly, aside from the withholding of payment for time spent at the employer's disposal, all international forced labour legislation says that the work must be involuntary, which it is as she was somewhat tricked into believing it was paid and only once in a while, and there must be a presence of threat - in this case loss of her job, which she can't afford.

Another hotelier told me that the director is an advisor to the hotel federation on salary manipulation and is always brought in to sort hotels in financial difficulty. This makes one believe he must be working within the limits of the law, and the fact that he has been doing it since the celtic tiger years and no one caught onto it(????) But at the same time I can't get my head around how it COULD be legal, especially for the parttime workers.

Is there anyone out there who can help to clarify the legality of this conundrum please?