Can I use Spain Schengen visa to travel another country during those 6-months?

I’m hoping to visit Barcelona this June and I’m trying to get an appointment to apply at BLS Ireland since a month but there are no appointments. Assuming I get an appointment and I am able to get the visa am I allowed to travel to let’s say Italy or France during those 6-months of Schengen visa? I don’t have plans as of yet so there are no tickets or booking for me to show during my appointment. It all depends on my work and other plans to see if I fit another holiday. So since we get Schengen visa for 6-months, I’m hoping to visit another country if I can. Is this possible? Thanks