holidaysong is correct in that any Dundalk fan, irrespective of location, could buy an away ticket for Windsor Park and make independent travel arrangements. I bought ticket-only in person in Oriel Park without any queries and subsequently drove to Belfast for the match. There was also an online option of ticket only or ticket / bus combo. The club did express a preference that supporters would travel on an official bus, but it was not compulsory.
The restrictions and disruption imposed on Linfield fans for the return match are a different story and were bizarre.