(Sorry for the late update, I have been too tired to write something)
In a shine of light Shirou appears before his master with his eyes closed and speaks the words "I have come to answer your summons master". Shirou then opens his eyes and widens them in shock, in front of him is his father Kiritsugu Emiya and who he assumes is Irisviel next to him.
"So this is King Arthur" sighs Kiritsugu as he closes his eyes and creases his forehead, "you're looking a lot more different than what you were portrayed as". Shirou with his white hair, steel grey eyes, red and black armor, and standing at 6ft 5in made him stand out from what Arthur Pendragon was portrayed as.
Shirou thinks to himself for a moment before explaining, "The legends have a track record when it comes to misidentifying figures of legend, an example would be Mordred and Gareth who are both women." This shocked both Kiritsugu and Irisviel who were just told that the history they studied may not be as reliable as they thought.
Kiritsugu may have to rethink some of his plans and ideas when it comes to tackling the other masters if he recognizes the servant, but he swiped the thought away and inquired on Shirou's abilities. Shirou let him know about his projections, which he disguised as duplication magic, and its ability to stay in the world almost indefinitely if it's not anything magical.
Kiritsugu, wanting proof, presented Shirou with a gun from his pocket, and watched in wonder as Shirou duplicated the one pistol into 20, all floating in the air. He was even further surprised when Shirou demonstrated that the guns were fully loaded when he fired one of them. While not fully understanding what was going on, Irisviel was very happy for her husband Kiritsugu that he summoned a servant that would be agreeable with his tactics, especially since he had no aversion for guns.
Satisfied, Kiritsugu let Shirou dismiss the weapons and eagerly led him to his armory where he presented Shirou with multiple weapons, all of which Shirou duplicated fully armed. Kiritsugu then went on to let Shirou know of his plans to have him pretend Irisviel is his master while he hides in the shadows to target the enemy masters.
When Shirou quickly agreed to its effectiveness, Kiritsugu questioned him on this, gaining Shirou's attention, he calmly asked "why are you so willing to follow these orders? Every story I've read on the Arthurian knights describe them as larger than life people who upheld chivalrous ideals without compromise. The type of people who would hate what I'm doing and protest every step of the way. Why are you not upset?"
Shirou took a minute to deliberate before answering, "chivalry doesn't win wars. When fighting a war, the only thing that matters is survival and victory, chivalry is for when the battle is won and peace has been made. Before I was a king, I was a soldier, and there is no place for chivalry on the battlefield." Such an answer from Shirou shattered Kiritsugu's preconceived notions on knights and chivalry and he gained some respect for Shirou and his realistic outlook.
With the talk over, Kiritsugu dismissed Shirou to Irisviel's side and went to Illyasviel's room, where he bid her goodbye and left the castle for the flight to Fuyuki Instructing Irisviel and Shirou to come to Fuyuki in 2 days. In the 2 days Shirou spent with Irisviel, he just got to know her and her attitude while spending time with Illyasviel, learning new things about both that he didn't know while watching the shows.
After those 2 days, Irisviel and Shirou bid Illyasviel goodbye and left for the airport on the almost 12 hour flight for Fuyuki, Japan. When they arrived, they spent time driving to the Emiya manor with Shirou driving the car very well, though passed it off as his Riding skill. They arrived and Shirou felt a sense of nostalgia when he looked at the place, carefully hiding it from Irsiviel.
They spent 30 minutes getting themselves settled in and going to the shed where Irisviel used chalk to place the array that would be used to summon Artoria a decade later. Irisviel then asked Shirou if he could take her somewhere, prefacing that he didn't have to if didn't want to, but this only reminded Shirou of Sakura so he quickly agreed and drove Irisviel to the ocean so she could experience it for the first time.
Shirou and Irisviel enjoyed gazing at the moon and chatting, though Shirou was preoccupied with thoughts about the confusing Moon Cell HGW and his disdain for vampires. Sensing magical energy kilometres away, he and Irisviel identified it as a servant and Shirou eagerly anticipated facing Diarmuid, familiar with his fighting style. They prepared and drove to the docks, where Shirou smiled at the sight of Diarmuid when they arrived, explaining to Irisviel his excitement to test himself against heroes from another era.
They exited the car and before Diarmuid could speak, Shirou projected Caliburn and performed a Mana Burst, appearing right in front of Diarmuid, catching him by surprise with a stab at his heart. Diarmuid was no slouch, even when caught by surprise he was able to bring his spear in front of him and maneuver the stab into his shoulder instead of the heart. The damage was quickly healed by his master but Diarmuid was given no time to adjust when he had to block a series of stabs and slashes coming at him.
He blocked, parried, and dodged every attack he could, responding with his own set that Shirou blocked with some difficulty before tracing Diarmuid's spears and understanding his fighting style, adapting in battle to better pressure Diarmuid. Shirou made sure that every blow of his sword was either blocked or met against the parts of the spear that weren't the tip, which would undoubtedly remove his projection of Caliburn.
Diarmuid saw the uptick in skill and ability, and noticed with annoyance that the unnamed saber servant (Shirou) was avoiding the tip of his spear, this forced him to use his Mind's Eye (True) and Knights Strategy to its full abilities.
Shirou who also had Mind's Eye (True) was not about to be tricked like Artoria so Diarmuid could land a few blows that would be fatal for this fight. This led to an intense exchange of blows, the clashes could be heard everywhere and the surrounding area was being destroyed by their power. This gave Shirou the fight of his life, exciting him, but also worrying him as his Mana Burst was starting to run out and the end of it would cost him the battle.
To avoid this, Shirou activated the full use of his Mind's Eye and found the most viable path to land a blow on Diarmuid. The only way to land the blow was to feign weakness so Shirou allowed himself to take a few cuts and bruises here and there to feign defeat. Always making sure to parry and block the blows that would cause him the most harm, by the end of it his appearance was a scratched up one.
Shirou's Mind's Eye let him know that now was the time since Diarmuid would now be confident in his victory. Ready, Shirou lunged forward, tracing a dagger and allowing himself to be stabbed in the gut by Gae Dearg, Diarmuid smirking in victory. Irisviel yelled in concern, and Diarmuid was distracted by his sense of victory, this moment of distraction cost Diarmuid as Shirou was able to nick his arm with the dagger, and severed his contract with his master and mana battery.
Diarmuid was bewildered by the cut off of his magical energy and was further surprised when he vomited blood, thinking it was a magic attack, Diarmuid flooded his body with mana to flush it out, this only served to worsen the damage. Shirou had used Diarmuid's bewilderment to project another dagger that severed his mana pathways and bound them together improperly.
This caused Diarmuid to convulse as his nerves and muscles spasmed and his organs popped like balloons. All that Diarmuid could do was lay there in complete agony as his body faded away into death. Unwilling to take a chance, Shirou projected Gae Buidhe and stabbed Diarmuid in the head, killing him and ending the greatest threat of this war in quick fashion.