OK, I'm new to this off-roading milarky but I'm wondering what I need to do or are the forces of nature against me.

*I have Grabber UHPs on the front and Pirellis on the rear, which I know are not ideal off-roading tyres.

I own some land in Ireland, which one day might be an MTB trail park. Some of it is quite steep in places. One of my current tasks is to keep the growth back, so that its accessible. With that in mind, have strimmed the paths. Being lazy, I don't want to carry all the strimmers/hedge clippers/spades/fuel etc up the hills all the time, so elected to drive.

Day 1, freshly strimmed, dry and I go straight up what is probably 35 degree hill. Overnight it rains and not a chance the next morning.
That evening, when it has dried a bit, take RRS up 25 degree hill with no issue. Next morning, after rain, not a chance.

I had it on Snow/Mud and Low Ratio + Raised

Should I be using any different settings? Would AT tyres make any/much/all the difference? Any advice would be great.

Otherwise I have to use my tracked dumper which storms it but is really slow!

AT tyres would of course increase the traction provided. Assuming you have 19" wheels, then the Goodyear Duratrac are the most rugged you can get in std 19" guise. Although they are 255/55/19 not 255/50/19.

I have these and they go pretty much anywhere so far.
Not overly noisy on road either. but road tyres, whilst good, will easily get clogged up with a little rain on a dry track. Bodsys Brake Bible
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Tyres, tyres, tyres......ATs will defo make a difference  Visitor from fullfatrr.com
* GONE. 2012 RR Sport HSE SDV6 Orkney Grey. Now being piloted by RRSTDV8
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The issue is wet grass. AT tyres will help but I've seen 4x4 with luggy MT tyres struggle on wet grass where the top layer of soil is also wet - the mix of those two can feel like driving on ice.

One little tip - switch off the DSC. This will kill the throttle if it detects sideways movement such as when on a slippery climb. It usually occurs at just the moment when you actually need some power to maintain momentum. Obviously you need to be careful if this is switched off (although it's never fully off even when selected off on the dash button).

Make sure HDC is switched on - it will help control your descent in reverse should you have a failed climb attempt. Reverse straight down too - don't try and turn across the slope or you risk rolling the vehicle. Oh, and if you start to lose traction during the climb, maintaining a constant throttle pedal will give the traction control system a chance to work. If you back off the pedal or mash it madly you'll prevent the control system from being able to help. Sorry if you know those points already but it's worth reiterating - especially the bit about rolling.

You might also want to try running in "rock crawl" on that slope. This will generally start with the centre diff locked which helps that initial getaway and approach on to the slope; it also gives finer initial throttle control than "snow" which dulls the response to the first portion of throttle pedal movement. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Would staying in high range not help ? Low range can create so much torque that it's no use on wet grass.
This was the advice I was given by an LR experience center driver. 

"Roads ? Where we're going there are no roads." Back to the future.

Staying in high range can help but......

If you need X amount of torque to get up a hill then nothing you can do that will change that. High range simply reduces the "snatch" of torque which can cut any traction you may have. There are two approaches - gentle throttle and trying not to brake grip or gung-ho and make it spin (DSC off). Always try the gentle approach first

Tyres will make a huge difference but.......

Even a tractor on new tread will struggle with heavy drawbar loads on wet summer grass. Winter grass is much easier to deal with but you'll make deeper ruts.

Sounds like a fun problem to have