What would our infrastructure look like if we had stayed in the UK?

Yep, just look at how the Border Commission worked out.

There was lots of publicly-funded rural housing built in in Ireland in the last few decades of British rule, and most of it was isolated houses, each associated with its own farm, rather than housing clustered in villages and towns. This was regarded as more efficient (each farmer on his own land), given the model of land purchase and distribution the Land Commission was following. Housing for landless farm labourers was similarly scattered.

This policy might have changed in later years if British rule had continued. On the other hand, it might not.

I think too many people compare their small little part of Ireland to London, Liverpool, Leeds etc.

My experience of England outside London was all roughly 90k cities which ironically is exactly the size of my home city Limerick.

Comparing the 2 I am very thankful for independence even if it took a bit of time to get there. We go on about their better public transport but again in those cities it's crap and a privatized mess. Cars are felt to be required same as here.

Certain things are better like I'de rather spend my money on council tax over private health insurance and GPs but their overly planned cities have been franchised into oblivion.

Another corollary of the airline industry that the Irish have successfully developed is the duty free and other shops in Russian airports when the Iron Curtain fell. That expertise has also helped it make inroads in other markets worldwide.

I also think it was worth the wait for Ireland to get the motorways that it has Today. They are really state of the art and on average, smartly insulated from residential areas throughout the country.

In terms of infrastructure development in Ireland-within-the-UK, it should be born in mind that IWUK would likely be notably poorer than real-world Ireland, since we wouldn't have been free to pursue the economic policies that have served us so well over the past 60 or 70 years. (In speculating how IWUK would fare economically we have a useful real-world control: Northern Ireland.)

So, infrastructural development would have been dependent to a material extent on financial transfers from GB. Which would certainly have happened to some extent, but I suspect not to such an extent as would replicate what real-world independent Ireland has been able to finance.

At the risk of oversimplifying, probably the financing of infrastructure in IWUK would have been better during the first fifty years of independence, and worse in the second 50 years. So we might have had some advantage in the kinds of infrastructure that the UK government was financing in the period 1920-1970. But for much of that period the UK government wasn't financing a lot of infrastructure even in GB — they don't talk about "interwar stagnation" for nothing — and most of what they did finance would be now be in need of replacement, if not already replaced.

We would most likely still be using miles instead of kilometers. Other than that, I would hesitate to hazard a guess!

Haven't seen it mentioned on here yet, but likely that Cork would have had a major role as a Royal Navy base in WW2 hosting Western Approaches escort forces(similar to its role in WW1). This likely would have made it a target for the Luftwaffe during 1940-1941..

There might well be Stormont.

It's a historical speculation so you can write it how you like, but the Government of Ireland Act 1920 provided for home rule within the UK, with separate parliaments and governments in Dublin (for 26 counties) and Belfast (for 6 counties). Because SF boycotted the Southern Ireland parliament and government they never functioned. But if they had functioned, we might very well have ended up with Ireland within the UK, but with separate Dublin and Belfast administrations.

Has Sligo grown less than Enniskillen?

Yes the social services end of things would have developed more quickly. But you would have ended up as NI is now, with big cuts coming up and limited private sector opportunities.