Hiya, Dublin City Council can supply you with a map of what’s going on underground – you should be able to request a public drainage map free of charge if you send them an email: email@example.com
If the line does run in the back garden, then, according to this:
It doesn’t sound like you would be able to build over it; even diverting it would be difficult to do.
Any neighbours who have built an extension in their back gardens should have some insights to share.
Hope this helps!