I'm also from the area, and while I'm not extremely familiar with Castlelands, I'm going to give you a few comments on property in Balbriggan....
Foreign Nationals
Yes, there are foreign nationals living in the town. Why should that be a problem? As property prices haven't been inflated as much in Balbriggan as other areas in North Dublin, the market has been affordable for first time buyers and those who would typically struggle to get a mortgage. I think that you will find that many of the foreign nationals own their own homes and don't rent at all. For example, and I live in the area - on my row of houses alone (there are 6), 3 of the houses are owned by foreign nationals.
Anti-social Behaviour
As with all parts of Ireland, there is always going to be an element of anti-social behaviour. The issue is what the gardai are doing about it. In the past I have had problems in my own estate with anti-social behaviour but with the help of the local Gardai, Council and the PRTB (for rented houses), it has all calmed down thanks be to goodness!
How well an estate develops in my experience is more down to the people who live there and their willingness to create a residents association, neighbourhood watch etc and work together. So for example if the kids are climbing on balconies, someone tells them to stop - has a word with their parents or whatever.
It's extremely difficult to say what way an estate will be like when it's not even constructed. From what I know, Castlelands is one of the better designed new developments - Moylaragh & Brega for example (and no offence to those who live there) were in my opinion very poorly designed and the houses are too close to the main roads in the area, with little secure open space for children and too far away from the main public transport routes. Castlelands on the other hand is perfect if you are a commuter as you miss the town bottleneck and it's still close enough to walk into town for the train or shopping.
5 years ago when I decided to move to the area, I knew I wanted to live in Balbriggan, just had to decide on an estate. I did a LOAD of research and that didn't involve asking the (biased) estate agent. I viewed the local development plans in Fingal County Council, I asked around other estate agents for their opinions on the estate, I drove up at different times of the day and night. I met with others who had lived in the area for years, and even just a few months to see how it was working for them. Do more research and you can't go wrong.