How to display a list of strings from a JSON file in a datagrid?

Object Class:

public class SIReport
{
    public string Sortcode { get; set; }
    public string Incident { get; set; }
}

public class Message
{
    public bool IsSIR { get; set; }
    public string Header { get; set; }
    public string Subject { get; set; }
    public List<string> Hashtags { get; set; }
    public List<string> URLs { get; set; }
    public SIReport SIReport { get; set; } 

}

I start by reading in a text file as a JSON

string jsonData = File.ReadAllText(@"C:\text.txt");

var messageList = JsonConvert.DeserializeObject<List<Message>>(jsonData);

The file looks like this:

{
  "IsSIR": true,
  "Header": "E123456789",
  "Subject": "SIR Subject Line",
  "Hashtags": [
     "#hasttag",
     "#hashtag2"
  ],
  "URLs": [
    "www.google.com",
    "www.stackoverflow.com"
  ],
  "SIReport": {
    "Sortcode": "98-23-43",
    "Incident": "N/A"
  }
}

Then I simply bind the list to the ItemsSource

Datagrid_Msg.ItemsSource = messageList;

It displays the strings no problem but under the columns with Lists it just displays "(Collection)". How do i go about displaying those lists of strings?