How on earth do you study any subject?
Study your set songs and works, choice songs and works, Irish music and your general study. After that just learn basic theory and be able to recognize instruments and be able to notate pitch. Then practice your instrument/s often.
Try practice as much unheard listening (aural skills) as possible. The only way to really do this is practice using your exam papers. For the Irish music, know the difference between jigs reels hornpipes slow airs and slip jigs, know their time sig and know how to write a bar of rhythm for each of them- this comes up every year. You need to know the difference through sound most importantly. Also learn the key features of Irish music such as flattened 7th, free rhythm, ornamentation etc. and most importantly, know how they SOUND. Also, make sure you know what every instrument sounds like as it is often asked to identify the instrument etc. Go through your papers and make a list of Irish essays you want to learn, write them up and learn them. The main thing with aural is to practice all the time in order to train your ear.
For the set works, learn the key definitions and practice every paper when it comes up! Honestly, you don't need to know the score back to front in order to do well. The questions are generally simple enough. Ensure that you know your different themes and sections of the works (as the first question is usually from what excerpt is the taken from) as well as time signatures, textures and main features. Eg. In Tchaikovsky's Romeo and Juliet,know the love theme, friar Lawrence theme and the strife theme. In piano quartet no.1 be able to explain key features such as canons and retrograde. Obviously you need to know more then just this but it is just an example. Try and write up condensed notes including the form, time sig, sections, texture, and key features such as compositional techniques and instrumental techniques. A lot of the questions can be answered simply by listening if your ear is trained enough.
Lastly, When it comes to composing it is all about practice. Make sure you know the basics ie. the scales, time sigs etc
There is always something that will be in the paper to try and trick you such as a minor key or an anacrusis so be on the look out for this. When you have a good grasp on the basics you can work more on making your compostion stand out.
The main thing is practice in the exam papers. It gives you a clear idea of how obtainable a H1 really is. Best of luck! x