Can the watch detect and remember episodes of irregular heart beat?

It could measure irregular beats and it does record a history of heart rates, but by default it only checks every 10 minutes or so. That means you'd have to be experiencing that when it happened to check your heart rate for any record to be made. If you tell the watch you are exercising it will check once a minute instead, but if you need it to check more frequently, you'd need a third-party app that monitored heart rate continuously. That would likely drain your battery extremely quickly...

You can take a manual reading of your heart rate at any time, but it's initiated by you and only measures for a set time period and gives the average rate, so it may not really indicate if your heart is irregular unless you test repeatedly or had a third-party app that is looking for it.