Hi Johnboy
If your sister was originally working two days and now she is on one day a week, then she is on short time notice. Her employer should of filled out the relevant paperwork for this (form RP?, i think, just google form for short time notice). Basically this form explains short time notice and you sister can only have her days reduced for 4 consecutive weeks and then after these 4 weeks she can apply for her employer to make her redundant or bring her back to her original 2 days. If you have a look on citizensinformation.ie then you will find more info on it.
Also as the previous posters said, she would only be able to apply for job seekers benefit if she is available for full time work.