I just saw an all-white painted 757 take off from Dublin (10.30pm). Its dark outside and most of the window shades were down so Im assuming it was not a passenger flight.
Your deduction was correct. It was TAY031W using unliveried 757 OO-TFA on the TNT cargo run to Liège
Departed directly south bang on 22:30.
Edit: it's a combi 757 and the windows ahead of the wing are permanently blanked.