Yes, and its quite good if you have the time. Don't travel in a month with an "r" in it, you are more subject to weather delays.
Can you imagine how much an airline would charge for a walk on fare? You can choose when to go, there are few/no restrictions on usage, no peak time off peak fare loading.
This is slower, but it works well enough on Dublin/DunLaoghaire - Holyhead.
Just don't get stuck in Crewe on a Saturday night ...