If you are using a Beckhoff PLC, Beckhoff has an OPC-UA server that is installed directly on the Beckhoff PLC running Windows CE/XP/7 (Windows is running on the PLC itself). You can have an Ignition HMI on Linux and access the Beckhoff OPC-UA server on the Beckhoff PLC.
Note that not all Beckhoff PLCs can support their OPC-UA server (the really simple / smaller memory ones are a no go). It however looks like it may be possible to use the OPC-UA server on one Beckhoff PLC to access the data on another Beckhoff PLC that isn’t capable of running the OPC-UA server on itself.
Without OPC-UA there is always Modbus TCP. You’re not likely to be thrilled with this option if you have a lot of tags, UDTs, etc. It can be done but it feels like you’re twisting someone’s arm–or getting yours twisted 