There is neither an Irish nor a European limit. The trains are designed to operate safely beyond crush loaded limits.
The safety authorities will not entertain complaints concerning overcrowding unless some form of incident was reported
We highly recommend booking online in advance to ensure a seat and a lower fare. The 12:30 was a sell out Tuesday and looks like it will sell out for Wednesday as well, still 10 euro though.
Overcrowding in an annual summer issue on the Westport route despite there being an extra train a day compared to past. Extra capacity is provided on the lunchtime train from Westport as a result.