It's not - in fact art has very little relation to architecture. Art might be better suited to Design. Architecture requirements for UCD are : "O6/H7 in English, Irish, Mathematics, a third language and two other recognised subjects" (
https://www.myucd.ie/courses/architecture/architecture/). So art would qualify as one of the two other subjects in terms of calculating points
The other thing to remember is that Engineering (and other sciences) don't require a language but they do require a "laboratory science" (which doesn't include technical subjects like material science or technical drawing). This means that if they want to keep open both Archictecture and Engineering, they will need to plan to do a science and a language for the Leaving Cert.
I'm not sure if any of this is helping - it sounds complicated but I find if you work back from potential college courses it can clarify i.e. If I need a language to apply to college I will need to do a language for the Leaving Cert. That means I must do a language for the Junior Cert OR I plan to take one up before the LC