First thing you need to do is identify how many UCAS points your qualifications will merit.. You will need to identify how your Irish qualifications translate into.
Then you need to identify courses by either using the UCAS site or soemthing like the Times or Guardian University guides. Alternatively you cna use HEAP university offers if you can find a copy as that details what the average offes were for each subject (libraries and careers services have it, maybe even in ireland if its popular to go to UK unis). There will be plenty of degrees with media in the name that offer diffeent aspects, so its a case of you having to do the research.
The better courses will have a higher ucas point tariff, although you should remember you pick the best overall uni for you and that might have to do with location, facilities, syllabus, cost etc. Nattow it down to 5 and put he one you most wnat to go to first. All the details on how to apply are on the UCAS site.
You cna normally go to open days to have a look round, but thats a case of looking at the website and your calendar. Once you apply, then you hopefully get offers and you cna have one main and one insurance offer. You go to your main if you meet the grades and the insurance if you dont meet the condition of your main, but do for the insurance offer.
The full process is all explained on the UCAS site with nice videos.