This works:

let ionLambdaFunction = aws.lambda.Function.get("ion-lambda-function", ionLambdaArn,
    {name: ionLambdaName}
);

And this does not:

let ionLambdaFunction = aws.lambda.Function.get("ion-lambda-function", null,
    {name: ionLambdaName}
);

Curious why you'd need to pass the lambda ARN and the name.