Not overly familiar with Belmont Park specifically but know the general Raheny area well.
Belmont Park was built about 20 years ago around the same time as nearby Ashcroft and Tuscany Downs so there may well be a lot of teenagers at the moment; however I wouldn't imagine they are any worse than teenagers anywhere and are probably better behaved than many.
The houses in Ennafort, Assams, Maywood etc definitely have more potential having garages, huge back gardens and they are really solidly built...dating from the late 50's and the early 60's. However you will pay more for them...at one stage in 2006 some of them were selling for 800,000 plus but I think you could get one at the moment around the 650,000 mark.
Not sure how well you know Raheny itself but it is a terrific place...great schools, wonderful transport links, lots of shops, banks etc and it has held on to its community/village atmosphere more than a lot of suburban Dublin.
I may be a bit biased as I grew up there but I really do think it is a secret gem.
Hope this helps a little.