Has anyone taken the fast ferry and if so was the trip smooth...I suffer from seasickness.
The fact (or not) that anybody has had a smooth sailing is no indicator that you would (or not) have one. You are speaking about the Irish Sea. I have never traveled on that precise route but I have crossed the Irish Sea many times. It all depends on the weather, but even more on the swell. Ships these days have better stabilizers than they used to but bad weather at sea will make a crossing more or less choppy. It will all depend on the season you are travelling and the weather a few days prior, the prevailing winds, and the weather on the day. I hope you have a calm sailing. Don't worry.