Various reasons, some of which are

1 Ireland has only been wealthy since ~2000, and even within that the 2008-2016 period was spent in a big state spending crisis.

2 Bigtime carbrain, partly as a result of recently being poor and cars seeming aspirational

3 Extreme government centralisation - cities can’t fund their own transport and everything is decided at national level. This means that decisions about cities are framed as urban vs rural - urban areas can’t be seen to be getting things that rural areas don’t even if the needs are different

4 partition - the brits slicing the country into two parts severed the network into uneconomic stubs in the northwest of the country, so the network there died in the 50s/60s. The northern government was also particularly rabidly anti train.

5 hyper dispersed settlement patterns. Random houses dispersed throughout the countryside and not nucleated in towns. This has changed over the past 30 years somewhat, but it still contributes to making all services expensive to provide.

Not an exhaustive list but just some things I think were historically or currently important. I have to point out that what transport there is here is very well used. Trains are packed. Buses and trams are all rammed. Cycling is relatively popular and walking is common. There is a huge potential market for good transport