External language server IRIS to IRIS?

Here's how to do it (sample code to transfer files over iris connection):

/// Get IRIS connection object
/// set iris = ##class().GetIRIS()
ClassMethod GetIRIS() As %Net.DB.Iris
{
    Set host = "host"
    Set port = 1972
    Set ns = "%SYS"
    Set user = "_SYSTEM"
    Set pass = "***"
    Set connection = ##class(%Net.DB.DataSource).CreateConnection(host, port, ns, user, pass)
    Set iris = connection.CreateIris()
    Quit iris
}

/// Transfer one file from sourceFile to targetFile on iris connection.
ClassMethod Transfer(iris As %Net.DB.Iris, sourceFile As %String, targetFile As %String) As %Status
{
    Set sc = $$$OK
    Try {
        Set stream = ##class(%Stream.FileBinary).%New()
        Do stream.LinkToFile(sourceFile)
        Set var = "%stream"
        Do iris.ClassMethodVoid($classname(), "InitStream", var, targetFile)
        While 'stream.AtEnd {
            Set chunk = stream.Read($$$MaxStringLength-1000)
            Do iris.ClassMethodVoid($classname(), "WriteStream", var, chunk)
        }
        Do iris.ClassMethodVoid($classname(), "SaveStream", var, ##class(%File).Attributes(sourceFile))
    }
    Catch ex{
        Do ##class(%SYS.System).WriteToConsoleLog("SuperServer Copy failure in Transfer:" _ ex.DisplayString())
        Throw ex

    }

    Quit sc
}

/// Initialize stream for subsequent write requests and place it in var.
/// var must be a global variable (start form %) 
/// file is created or truncated if already exists
ClassMethod InitStream(var As %String, file As %String)
{
    Try {
        Do ##class(%File).Truncate(file)
        Set stream = ##class(%Stream.FileBinary).%New()
        Do stream.LinkToFile(file)
        Set @var = stream
    } Catch ex{
        Do ##class(%SYS.System).WriteToConsoleLog("SuperServer Copy failure in InitStream: " _ ex.DisplayString())
        Throw ex
    }
}

/// Wrile string into a stream initialized by InitStream 
ClassMethod WriteStream(var As %String, string As %String)
{
    Try {
        Do $method(@var, "Write", string)
    } Catch ex{
        Do ##class(%SYS.System).WriteToConsoleLog("SuperServer Copy failure in WriteStream: " _ ex.DisplayString())
        Throw ex
    }
}

/// Save stream initialized by InitStream. 
/// Optionally sets file attributes.
ClassMethod SaveStream(var As %String, attributes As %String = "")
{
    Try {
        Set sc = $method(@var, "%Save")
        Set file = $property(@var, "Id")
        Kill @var
        Do:attributes'="" ##class(%File).SetAttributes(file, attributes)
    } catch ex {
        Do ##class(%SYS.System).WriteToConsoleLog("SuperServer Copy failure in Savetream: " _ ex.DisplayString())
        Throw ex
    }
}

Use ClassMethodValue to get a scalar value back. Use json for complex type transfer as objects are not supported. There are other methods corresponding to APIs in other languages.

Also please note that this class (technically only the callee *Stream methods but save yourself a headache and just copy the entire class) needs to be present on both nodes.

Finally, remember that callee methods must produce no stdout/stderr writes, since the io is bound to the iris connection itself and it cannot disambiguate stdout.