An Irish heatwave, as defined by Met Éireann, is a period of at least 5 consecutive days where a station has recorded a maximum temperature of 25.0°C or greater.
The definition has been debated to death time and time again over its flaws and just how useful it is to studying the history of heatwaves in Ireland. Some heatwaves may have been missed out by a single 24.9°C figure in the middle of the period for example as it did not satisfy the criteria - late May and early June 2020 was a recent occurrence of this at several stations. Another issue might be the consecutive period of 25°C days lasts for 4 days instead of 5. However, whilst there's no getting away from these very apparent issues, the definition ensures there is some sort of consistency and is straightforward to understand as compared to if one was to take an anomaly from average approach similar to the WMO definition of a heatwave which states at least 5 consecutive days must be 5°C or more above average. This approach would be inconsistent and complicated however as the anomaly would be biased to the relativity of the station and could also be dependent on the climate period used.
Analysis of available Irish station data suggests there have been heatwaves in 1921, 1934, 1947, 1955, 1957, 1959, 1972, 1976, 1983, 1989, 1991, 1995, 2003, 2005, 2006, 2012, 2013, 2017, 2018 and 2021. If a decadal average were to be calculated from these years alone, it would be 1.9 so roughly between once or twice a decade. However, it must be noted that the 1960s decade had no heatwaves and were notorious for having cool summers. Also, the further back we go, the less useful this average gets as the lack of stations would have played a part in less heatwaves being realised as compared to the modern day likely skewing the average. Climate change may also be playing a significant part.
There was one heatwave during the month of May - 2012, whilst there were two instances of heatwaves during September - 1959 and 1991. The rest of the Irish heatwaves in this analysis occurred during the meteorological summer months of June, July and August. The years 1976, 1989, 1995 and 2006 recorded at least two individual heatwaves in separate months and spells at one or more Irish stations.
The record for the longest heatwave in Ireland is 18 consecutive days at Kilkenny (Greenshill) (County Kilkenny) from 24 June to 11 July 2018, followed by 14 consecutive days at Birr (County Offaly) and Ballybrittas (County Laois) from 14 to 27 August 1976. The hottest heatwave in terms of mean maximum temperature was 29.8°C at Dooks (County Kerry) from 26 to 30 June 2018 whilst Boora (County Offaly) had a mean maximum temperature of 29.4°C from 27 June to 5 July 1976 including 5 consecutive days of 30°C from 29 June to 3 July 1976. This is the only known instance in Ireland when 30°C was achieved for a 5 consecutive day period whilst Kilkenny (County Kilkenny) achieved 30°C for 4 consecutive days from 12 to 15 July 1983.