First proper Road bike - few questions and options please?

I've not read through all the comments but definately look at the saddle and getting measured for one. This could make a whole lot of difference in terms of ride enjoyment and numbness!

Also, the amount you are going to spend, is it worth it and how often are you going to use it?  

180k can be a long way and if it ends up being a beasting for you, it might put you off riding again so give yourself plenty of time on the bike beforehand.  A few hours on the bike will give you an indication on where your weaknesses are biometrically. 6 hours or so on the bike will expose these. 

Finally, good luck!

You say that the bike "won't be for commuting", what is the reason for this?

I am going to make the assumption that as you are wanting to buy your "first proper road bike" and the impetus behind this being "a 180k cycle (hilly, Irish roads) in the summer" that you have not done long distances previously? If that is the case then I would strongly advise you to look into the possibility of commuting by bicycle to get as many extra miles under your belt before the event as you can. I really think that you will be grateful for this in the end.

A few thoughts to add to those above.

Re: brakes. Rim brakes can be perfectly good and requiring little adjustment and maintenance - if you ride dry roads! I don't know how fair weather a rider you are but Ireland is fairly damp, so if you're going to be out on wet roads a lot then discs may be the better option for you. Their performance will be less affected, and they won't grind down pads and rims so quickly.

On the 'feel' of a road bike, it probably is more arse-up/head down/arms out than you're used to, and narrower. But it's easy to flip (or even replace) the stem for a more upright riding position in the early months - you can flip it back when you're more used to it.

Likewise, if you're relatively broad shouldered, road bars are also pretty narrow - typically 42cm on new bikes. I swapped mine for 46cm bars and felt better for it. Also, look for shallow drop bars that don't need you to get so low when on the drops (most new bikes have these). And reach too, as veloUSA says.

I tend to agree with hawkinspeter than an expensive bike fit at this stage is perhaps too early - both bikes and bodies are adjustable, and the latter will definitely adjust in the early stages of road riding - just by getting used to it. With some online research of your own, trying out a few bikes and asking sensible questions you can get pretty close.

It might be an idea to borrow or hire a suitably-sized bike for a few rides (further than a shop would allow a test ride - although that's also a good idea). You'll get an idea what feels right and what aches after an hour or two, and can then make a slightly more informed decision about whether you want a stretched-out racer, or a more upright endurance bike.

On groupsets, 105 is excellent - but Tiagra is very much like the excellent 105 of a couple of years back, with the bonus that 10 speed replacement chains and cassettes are much better value than 11 speed. Some look down a little on Tiagra, thinking 105 is the basic level for "serious" roadies but that's bollocks.

Cannondale Synapse would be excellent for a first road bike. They're considered to be a "comfort" geometry, but they're at the more racy end of that category.

I'd agree with guinom8 about the usefullness of a bike fit, but I think it'd be more worthwhile to go for a bikefit after getting a bit more experience. Ideally, you could have a cheap (£50?) basic bike fit with an expert just eyeballing your position and giving you sizing recommendations, but it won't be that useful as you wouldn't have settled into your cycling position at that point. If you ride your bike for a month or two until you feel settled, a bike fit would then highlight more detailed changes such as saddle position etc.

With brakes, my preference would be for hydraulic disks first, rim brakes second and cable disks last. I'm a big fan of hydraulic disks and I think they're absolutely brilliant although can be very squeaky in the rain (as opposed to rim brakes that don't work in the rain until you get the water off the rim - typically one revolution of the wheel). I haven't tried cable disk brakes, but they seem to need more adjustment and care.

What Velo said. 

I have a range of bikes of different types but i only do short hops so size doesn't really matter as much to me. If you're gonna do any big miles then make sure it fits and fits well.

No matter how much you spend on a bike if it doesn't fit well you'll grow not to like it and end up on the same route as many others have

Shop > ride > garage > shed > ebay...a well worn path...

Choosing your first road bike can be a doozy. Going by "every bike I've tried feels 'weird' just because its sharper to steer, different hand position etc which is a little hard to judge them by." it appears frame size needs deeper consideration. A brief write-up here https://www.bikeradar.com/us/road/gear/article/road-bike-sizing-what-siz...

But don't hold this as the gold method. I am 5'-7" and ride a 49 frame. My recommended size is 51/52.  Howvever, I have owned several 51/52 frames and always felt "stretched" out. This is why I went down to a 49. Suggest you ride a recommened size and one size smaller. Be sure to ask about the handlebar size (width) and reach. It will help answer "different hand position" you're experiencing.

Ride as many bikes as you prefer. Understand head tube height. A taller head tube sits you more upgright.

Buy the bike you like not what others like. AND, after purchase be sure to get a bike fit. Without one you're riding will be uncomfortable, with one a pleasure.