The following table was taken from data.oecd.org:
This link gives the Big Mac index.
Does this mean that Polish people in Poland have more purchasing power than Irish people in Ireland?
In other words, if we compare two persons living in both countries, is a Polish person enjoying more affluence?
In other words, if we compare two persons living in both countries, is a Polish person enjoying more affluence?
No. Big Mac index estimates the implied purchasing power parity between local currency and dollar not purchasing power of individual.
For example, if Big Mac in US costs \$4 and Big Mac in UK costs £2 the implied purchasing power parity is $\frac{4 {\\\$}}{2£}=2\frac{\\\$}{£}$, implying that the estimate of "correct" exchange rate (under assumption that law of one price holds) of USD to BPD is 2 (or often Big Mac Index is expressed as local currency to USD which would make it 0.5 BPD to USD: $\frac{2£}{4 {\\\$}}=0.5\frac{£}{\\\$}$). This, on its own, tells you nothing about relative affluence of people in different countries because you don't know what their income is. Generally Big Mac Index is primarily used as a rudimentary check whether currency is over or under valued.
Purchasing power parity is useful to convent all incomes to the same units. You cannot compare zloty to euro incomes directly since that would be comparing apples to oranges. However, in itself its just conversion factor that does not tell you much about welfare of individual in particular country. Also Big Mac Index is typically not used for this because it's less precise than more nuanced measures of purchasing power parity (PPP). Advantage of Big Mac Index is that it can be calculated faster since it involves just comparing costs of Big Macs.
The most basic measure of welfare would be GDP per capita which tells you how much output/income country has per person. Adjusting for purchasing power parity the GDP per capita of Ireland is currently 3x as high as GDP per capita of Poland.
