There's three main reasons for this:
1. Yield. Loads and income on the route would be rather inconsistent, being very event based. Its also likely to have less of your business traveller than typical successful routes such as New-York and Boston, as well as not having as many leisure as Orlando.
2. There are no U.S airlines with major hubs in LAS. Notice all the routes from Ireland to the US currently with the American 3, all to hub airports. LAS isn't a hub airport, so its very unlikely a US airline would start the route.
3. Lack of a market. Just look at the population of the UK. Look at the amount of flights from London alone, its guaranteed the market will be far larger in London hence the success.
Off the record then, I have noticed, maybe incorrectly that Irish travellers (the good kind) tend to fly to City Routes and the US alot more than our UK counterparts who seem very Leisure (Las Vegas, Orlando, Spain, Carribean) and Middle Eastern focused.
The above is not to say that LAS will never come, I'd say its in the pecking order alright, but there are more lucrative routes for the moment.