We flew from U.S. to Ireland recently and thought it odd that we were required to keep blinds closed at all time during flight.
We asked flight attendant why that was the case but she only said it's part of flight regulations. Why?
I bet it was a 'night flight'. One where all passengers were encouraged to sleep most of the flight.
Likely a meal, dinner or supper, soon after the start and a second, breakfast style, shortly before arrival and little or no service (unless asked for) the rest of the flight.
I do not like it myself, I love looking out and will not sleep anyhow during the flights. Having the blind opened a crack, enough for the person at the window to look out, has been allowed, but it is against some airline rules.
'Daytime' flights usually have the passengers free to open the blinds.
It isn't against any regulations.
The reason is that you are flying towards dawn, and at some point it will suddenly get very bright outside. With the blind open, the brightness will awaken everyone else in the cabin. Many people on your flight are going to work and this is the only chance they get to sleep before a long day of meetings in Dublin. Waking them up unnecessarily will make a lot of people grumpy with the cabin crew. Therefore cabin crew do often ask you to close the blinds or close them for you if you go to the loo or doze off.
This happens on day flights as well and is quite annoying if you are working or reading with the assistance of natural light.
A firm "I prefer the blinds open, thank you; but I shall remember to close them before I sleep" is usually sufficient to move the situation along. If not a discreet word with the purser may be necessary.