Recently I found a link on /r/dataisbeutiful to a choropleth entitled "How Diverse is the U.S.?". At first I looked at doing the same thing applied to Ireland but the data isn't all that interesting. Then I thought about applying it to political parties in Dáil Éireann.
Simpsons Diversity Index was developed to examine the diversity of ecological systems but can also be applied to human populations or any scenario in which there are different types like political parties, colors of M&Ms etc. I took the results of each General Election since 1918 from Wikipedia and applied the formula to get the diversity of each Dáil. The result can be expressed in a number of different forms and it is important to know which one is being used. The first is Simpsons Index which gives the probability that 2 individuals selected at random belong to the same party. Simpsons Diversity Index takes this number and subtracts it from 1. This gives the inverse, the probability that 2 individuals selected at random will belong to different parties. This is the number that I use (multiplied by 100 for plotting) and results in a higher number meaning the Dáil is more politically diverse and a lower number meaning the Dáil is less politically diverse.
The above plot shows two lines. At various times during the history of the Dáil some individuals elected to it have abstained from taking their seats. In the first two Dáils the elections were held to the British Parliament or the Southern and Northern Home Rule Parliaments but Sinn Féin treated them as elections to Dáil Éireann and as such were the only party to take their seats. The 2nd includes the results of Northern elections as these MPs would have been allowed sit in the Dáil and vote. In the 20's Sinn Féin and Fianna Fail abstained until Fianna Fail entered the 5th Dáil after a period of time and later Dáil's saw Sinn Féin or Anti H-Block TDs refuse to take their seats. The continuous line shows the diversity of the parliament elected by the people and uses the full election results. The dashed line takes into account only those who took their seats. Therefore the dashed line for the 1st and 2nd Dáil has a diversity of 0 as only 1 party took their seats. The dashed line for the 5th Dáil shows the figure after Fianna Fail took their seats. Independent TDs are treated as a single grouping if not members of a group.
The 32nd Dáil, elected in February 2016, was described at the time as the most diverse we had ever had due to the numbers of Independents that were elected. This Dáil has a score of 79.47% (the probability that 2 members picked at random are not members of the same party) and is the highest score since the 5th Dáil, elected in 1927. Both are marked by the 2 largest parties having only a few seats difference between them, a strong 3rd party and a large number of different groups, 10 for this Dáil and 7 for the 5th.
Ignoring the first 2 Dáils the least diverse were the 10th (1938-43), 21st (1977-81) and 19th (1969-73) with scores between 57% and 60%. All 3 are marked by large Fianna Fail majority governments and only 4 groups, Fianna Fail, Fine Gael, Labour and Independents, of which there was only 1 in the 19th Dáil.
High diversity has tended to favor Fine Gael; of the 7 most diverse Dáils Fine Gael led 6. Its predessor, Cumman na nGaedheal led 2, Fine Gael led coalitions won 3 and during the 27th Dáil Labour left its coalition with Fianna Fail to put Fine Gael in government. Of the 7 least diverse Dáils 2 were convened with only 1 party while Fianna Fail won outright majorities in 4 others. The 24th Dáil (1982-87) saw Fine Gael and Labour form a coalition to keep Fianna Fail out of office by just 1 seat.
Periods of low political diversity saw large Fianna Fail majorities returned but diversity has been increasing since the late 80s with new parties coming and going. The financial crisis saw a collapse in Fianna Fail support and Fine Gaels performance in the last Dáil has seen their support fall. From this has risen a diverse but fractured Left and a surge in support for Sinn Féin. This could result in the end of the dominance of the 2 historically large parties if these smaller parties continue to grow and the Dáil stays at this level of diversity. A lower figure could see Sinn Féin join Labour in propping up 1 of the big 2 in coalitions or an even lower figure could result in a return to the old status quo with either Fianna Fail returning to prominence or Fine Gael holding on to its position as the largest party in the state and forming governments with a handful of Independents. Recent opinion polls suggest that the second option might be the most likely and at the moment a return to the days of 1 party gaining more than 50% of votes cast at an election seem a long way off.