Well the minimum weather for VFR(Visual Flight Rules) which you will do most of your training in differs according to the airspace your flying in. For example if your in Cork- Class C you need 5Km vis and 1500 Feet cloud base. However most schools would not take you up in that, its a waste of your time and money. Wind is another story, anything above 20 knots and your pushing it. Even then when you start your instrument rating even though you can legally fly with a 200 foot cloudbase on a ILS it's not a good idea, if you have an engine failure you need to be able to glide to a field and the wind can mess up holding patterns etc. Also there is roughly half the year not flyable in Ireland.
So basically in Ireland you need lots of patience and will power. It is tough work and some days you wish you never started! Don't see how you are going to get it done in 18 months, took me that to get my PPL(partime). Took me 5 months to get MEP,CPL and MEIR(fulltime). Forgot to mention it took 6 months to do the most "fun" part the ATPL written exams.
By the way which school are you going to?