A good school doesn't allow that to happen. You seem to have some odd and fixed ideas. At DD's school there were as many girls as boys who took STEM subjects at A level.
Let's face it, most schools are not good schools or even good enough schools. One third of British school leavers are functionally illiterate.
Your observation of STEM preference in your DC's school is not reflective of the wider UK picture.
How do you motivate an unmotivated student to access these resources?
You work hard to engage them in a conversation where they articulate their fears, difficulties, and issues about their self image.
You set up a university admissions system that rewards effort in challenging subjects.
You explain to them that getting into (for example) art school, and becoming a puppeteer, requires effort in areas not directly related to art, and you try to support them in every way you can to get over this hurdle. You can remind them that they will need to do very mundane and not at all creative chores like keeping accounts, filing taxes, negotiating studio rent, etc exactly as if they were setting up a beige paint shop, and that simply adulting means they will also have to maintain a car, shop for groceries, cook, keep a clean home, manage their time sensibly, and lots of other stuff completely unrelated to creating puppets and doing puppet shows around the country.
www.ncad.ie/files/undergrad_files/NCAD_2021_Minimum_Entry_Requirements_.pdf
If all of that fails, you recalibrate expectations and direct a student's attention to a third level institution whose entry requirements are not as rigorous, or help to figure out a more circuitous path to a certain career.
If a student is experiencing a deep crisis of confidence and is lacking any sense of direction, maybe find a therapist.
Silk purse/sow's ear
Finding the energy to work at a dull subject is a function of maturity, and understanding that not all students are going to find the subject you teach riveting is part of the role of a good teacher. If you were truly working hard and still only managed to scrape a pass, then your problem may have been an ineffective teacher providing insufficient feedback, a lack of confidence on your part, or an inability for some reason to seek help.
Learning to learn is a vital element of education. You don't learn to learn very efficiently if you stick to your comfort zone.
Fwiw, here are the subjects available (subject to teacher availability and timetable constraints) for the LC in Ireland, and a description of the levels available too.
en.wikipedia.org/wiki/Leaving_Certificate_(Ireland)
It's not the brutal, one-size-fits-all regime some may imagine.
There are many avenues to a degree:
www.qualifax.ie/index.php?option=com_wrapper&view=wrapper&Itemid=15?Mainsec=courses&Subsec=course_details&ID=21743