I haven't lived in Bayside so I can't comment on Bayside.
I did live for many years in Santry. I would agree it feels like an area that people pass through...and they do, in their hundreds everyday in their cars. I would advise anyone looking at Shanowen, Shanard, Shanliss to have a look there at peak hour to see what the car traffic is like...it can be pretty horrendous at worst and continous at best. Always find people trying to reverse out of their own driveways or cross their road with their children have to wait for thru-traffic to pass by. As I say, at peak hours it is continous. A lot of people access the Ballymun Road route into town by coming off the M1 or Airport Road and driving through Shanliss, Shanowen etc. So there's that.
There are a lot of rented houses too given the proximity to DCU so you can be lucky or unlucky there. We had a rented house next to us and we had both very good and very bad experiences. I think the good ones were when he stopped renting to students. Some of them are like something let loose! If you're buying check out if house next door is rented.
Bus routes aren't bad if you're working in the Northside of city centre. But if you have to get to St Stephens Green etc then maybe O'Connell St is a problem. It was when I was there but maybe things have improved.
Shops are not great at all. Omni despite refurbishment and extension, there still isn't many shops on offer. There is a local Spar and Centra which have handy opening hours. The Esso station is down the road if you run out of milk late at night.
Cinema at Omni, so that might be handy for kids. There are schools in the area too. There is a park ten minutes at most from Shanowen. Playground for kids, that's nice. But I wouldn't think there is a great deal going for atmosphere or community spirit. I didn't find any! I had lovely neighbours next door but I hated the traffic. Lost one of my favourite cats to it.
That's just my tuppence offfering.