Firstly, Glanmire is North of river, all other quoted areas are South. there is a differential in prices N v S.
Is it a good investment? Who knows. You will need a roof over your head though ,along with GF and children if they come along. First house in my view and it is a personal view, should be in a place that you are prepared to live yourself, comfortably.
Will prices fall/crash? Again who knows. This has been forecast for many years, but has not yet come to fruition. If one keeps repeating it often enough, it will come true sometime. If prices fall in 2025 will whoever forecast it in 1993 be saying, I told you so? You want to buy, then take a chance and buy. You must risk to get through life.
Rental market has dropped in the Student area. If you are hoping to get students, then you might have a problem. if hoping to get workers, you shouldnt in any of the areas you mention. Wilton will let to medical people, Grange to city workers and Glanmire to city people as well.
If you cannot service the mortgage without the house being full, I think you need to reconsider the venture. If full, will you have any privacy with GF, or will other tenants also have GF/BF staying the odd night? If all rooms are full, you will haev at least three other people buttin gin on your relationship.
At the end of the day, all areas mentioned seem good value. you will not get any 3 bed semi in Cork City under €320K. You have €100k to put in, so negative equity should not really be a worry if prices stabliise or take a slight fall.