December 1989 was one of three colder than average Decembers in Ireland during the 1980s for most places although localised parts of the east and southeast had a fairly mild to close to average (but on the mild side) December - the 80s was known for mild Decembers. Compared to December 1981 which had been the coldest December of the 20th century, December 1989 was far less remarkable.
For the CET, it was actually a relatively mild month with a mean temperature of 4.9c. So a bit of a strange month.
December 1989 fell nearly in the middle of an infamous period of mild to very mild Winters following a period of cold/snowy Winters through the mid 1980s (from 1984-85 to 1986-87). It was the only "coldish" month in the period if that makes any sense. It could have been quite a cold month if the pattern wasn't so messy. The 500mb height anomaly reanalysis of the month below shows that the Azores High is displaced with the jet stream attempting to be on a southerly track. However, it gets stopped by high pressure over the med and central Europe which is not good for a cold pattern. The high pressure that was there over Greenland and Iceland tended to be very weak.
In terms of 1951-80 averages, Belmullet had a temperature anomaly of -1.7c below average in December 1989 whilst Dublin Airport and Rosslare had temperature anomalies of +0.5c above average. The mean wind direction during the month was westerly in the south of the country due to the positioning of the jet stream whilst the north tended to have easterly winds which explained the temperature distribution.
The first 10 days of December 1989 were very anticyclonic with plentiful high pressure but this led to frost and persistent fog for some. In urban areas like Dublin similar to November 1988, there was instances of smog. The Met Éireann monthly weather bulletin of December 1989 gives a good account on this smog incident (whilst I did a post on the November 1988 one). Birr got down to -7.0c on 1st December.
The weather turned more unsettled after the first 10 days with low pressure setting up shop to the south and east of Ireland whilst at the same time, an easterly flow developed to the north. These low pressure systems brought a lot of rain to the south of the country including to counties Kerry, Cork and Waterford. Cork Airport recorded 163.0mm (its entire monthly total for December 1989 was 239.0mm) alone between 11th-16th December 1989 including 34.4mm on the 11th, 48.4mm on the 13th and 51.3mm on the 16th. Such large totals in a short period of time of course resulted in severe flooding.
The low that brought the heavy rain on the 16th December was very deep indeed, see the synoptic below for 0000 GMT on 17th December 1989. This storm bottomed out at a pressure of 941mb. Cork Airport bottomed out at 942.5mb, its record low pressure observation since its records began in 1962 and still is to this day.
Another storm on 21st December brought 20-30m waves in the Bay of Biscay and these waves crashed against the La Jument lighthouse in France. The waves smashed through the lower windows of the lighthouse, ripped the front door, flooded the tower and washed away the furniture.
Christmas Eve brought in another deep storm from the Atlantic though thankfully its centre was a distance away from making landfall on Ireland so was not a 16th/17th December scenario. Nevertheless, the winds still blew very hard with the windiest conditions of the month experienced including a 78 knot (144 km/h) gust at Belmullet. It became the mildest day of the month with long fetched southwesterly winds. Dublin Airport had a maximum temperature of 13.7c on the day.
It gradually quietened down for the end of 1989 as a southeasterly airflow developed with the year concluding on a benign note.
December 1989 got crazy wild very fast!