The picture below was taken on 22 October 2003 in Finglas, Co. Dublin.
My educated guess is that barely any of you remember what happened here in October 2003 - I never see this month being mentioned by any of us (including myself).
Well firstly, I would like to give some information on the month itself, October 2003. It was a colder than average month - and was the only month of 2003 to be cold. In many parts of Ireland, it was the driest October on record, including at Cork Airport where they had only 23% of their average October rainfall. Meanwhile, Valentia Observatory had its driest October since 1919. In contrast to the rest of the 25 counties, Dublin had a wet October. Dublin stations had between 150-190% of their average October rainfall but even saying this, it was not as wet as October 2002. I'll say it again, no other county was wetter than average for this month.
Now why was Dublin wetter than average unlike all the other counties? For a start, the dominant pattern for October 2003 was high pressure out in the Atlantic with a low to the east of Britain allowing the winds to come from a northerly direction. Normally in these kind of setups, the east is wetter than the west.
The other reason is because of the unusually heavy rain Dublin had from the 21st-23rd and the 30th. You can see Dublin Airport's rainfall spikes for each day of October 2003 (along with Valentia Observatory and Clones, Co. Monaghan) in this graph below from Met Éireann's October 2003 monthly weather bulletin.
Here's the synoptic chart for 22 October 2003, notice the low centred over the English Channel bringing the winds in from an easterly. Casement Aerodrome had a daily rainfall of 51.6mm on this day, its wettest October day on record until 24 October 2011. Maximum temperatures were under 10c on this day along with minimums getting close to or below freezing, so this was highly unusual. With the colder temperatures and easterly winds came some snow - as seen in the picture I posted. These heavy falls on the 22nd in Dublin were associated with continuous thunderstorms bringing a combination of rain, hail and snow.
To emphasise how cold it was, two days later on October 24th, -5.5c was recorded at Fermoy. At the same time, Kilkenny had a grass minimum of -10.9c, its lowest on record for October.
RTÉ Archives report/video on the thunderstorms, floods and snow in Dublin on 22 October 2003: https://www.rte.ie/archives/2013/1022/481962-thunderstorms-in-dublin/