There aren't many good invasion beaches in Ireland or the West of the UK. The US would need to seize a port like Cork and then build up air bases and deploy other forces the Irish road net even in the 1990's remained rudimentary compared to the UK; it wasn't until recently that road projects received serious investment (mainly by the EU).
The West of the UK has similar issues in that there are few suitable beaches in the South West or Wales, plus the road net in the 1940's was woeful consisting of one and two lane roads and rail both of which could be blocked with relative ease. Even in the 1960's the Motorway network was still limited west of Swindon and remained so until the 1970's by which time a lot more dual carriageways had been built. However, these still provided limited and easily blocked access to the rest of the UK.
luckily the US Army is really big on engineer units, from division sized groups to battalions assigned to divisions
construction is arguably the thing the 20th Century US Army is best at
but yes the relative poor development of Ireland is an issue, although it is still better developed than Iceland or for that matter anywhere in the Pacific during World War II, including a lot of Australia