Well it has been two years since I've been on that line to Westport and I'd say that the train is in line with the standards of most European operators, so no need to worry about being cold on board etc. They even have free Wifi!
You can indeed reserve and even select your seat online, but I've had mixed results with it. Between passengers who ignore the reservation and staff who seem to be unwilling to enforce the rules, I'm not sure it is worth the effort! Just be in plenty of time.