IDS Implementation

hi all,

my infrastrucuture area is implementing a IDS in our network.
i had a conception that an IDS had to be installed after the firewall in order to catch all intrusion tentatives.
However, the engineers are saying to me that the IDS has to be installed before the firewall.

I?m concerned that in case of intruder bypass my IDS, how could I know that he is trying to enter into my network? Would I be secure with this solution?

Could you explain this better to me.

Thanks in advance.
Neft Jr.