Mr. O'Reilly was an ineffective and God-fearing Irish cowboy builder played by actor David Kelly who Basil hires in The Builders. His shoddy workmanship and complete lack of knowledge in the field of building works has led to disaster several times, but despite this Basil continued to hire him because unlike professional builders he was cheap. Basil only ceased using O'Reilly after his last job almost resulted in the Fawlty Towers hotel collapsing.
As mentioned in The Builders, O'Reilly had been hired by Basil twice before in the past year, and each time it had ended in disaster. The first time, he had been tasked with building a wall, which is currently still lying outside the hotel in a pile of bricks despite Basil's constant telephone calls (as shown in A Touch of Class). The second time occured around November, when Basil had foolishly hired him to change a washer, despite him not being a plumber, resulting in no running water for two weeks. Because of these constant fiascos, Sybil has an intense dislike for the builder, greatly preferring the far more competent Mr. Stubbs.
In the episode, Basil and Sybil are going away for the weekend and decide to have some foundation work done on the hotel in their absence, more specifically blocking off the drawing room door and knocking down a wall to install a door leading from the lobby to the kitchen. Sybil sensibly decides to use professional builder Mr. Stubbs and leaves Basil to make the arrangements. However, while a highly competent builder, Stubbs is also expensive. Not wanting a huge bill to pay, Basil hires O'Reilly while Sybil is unaware.
Basil and Sybil depart the day the builders are due to arrive, leaving Polly in charge. However, Polly did not have a good night's sleep the day before, and asks Manuel to take care of the hotel while she takes a nap. Before she goes upstairs, Polly instructs Manuel to wake her once the builders arrive. When they do, Manuel cannot bring himself to wake the peacefully sleeping Polly, and his poor english prevents him from correcting the builders when they incompetently misinterpret the plans.
When Basil returns early to check on the hotel the next morning, he finds the lobby a mess: the drawing room door has been untouched and there is no kitchen door. Instead, a door has been built at the foot of the stairs and the dining room door has been blocked off. The horrified and livid Basil gets into a loud argument with Polly over who was responsible for the shambles, and after the blame falls on Basil for hiring O'Reilly in the first place, he goes into a panic attack over Sybil's anticipated reaction. Polly manages to slap some sense into him and tells him to call O'Reilly: he made this mess, so he should come and put it right. Once Basil has calmed down to an extent, he quickly gets on the phone and calls the builder. When O'Reilly begins making excuses, Basil tells him to arrive in twenty minutes with his dining room door or he would come over and shove a large garden gnome in him (The gnome Sybil had ordered which arrived before the builders did).
Because his men would not work on a sunday, only O'Reilly himself arrives. Because Sybil would be returning in four hours, Basil is frantic for O'Reilly to get started, but the laid back builder merely begins spouting some religious nonsense concerning Basil's constant worrying. Suddenly, Polly rushes in and tells them that Sybil has arrived early, having forgotten her golfing shoes. While Polly hides in the drawing room and O'Reilly hides in the bar, Basil desperately attempts to stall her, but fails in stopping her from entering the lobby and seeing the mess. Basil quickly comes up with a lie that Mr. Stubbs, despite being a supposed professional, had made the mess himself. Having seen O'Reilly's van outside, Sybil is clearly not buying it, but pretends to anyway. As Basil continues to tell lies, Polly makes a phone call from the drawing room, pretending to be Mr. Stubbs's secretary. Recognising her voice, Sybil hands the phone to Basil and finds Polly in the drawing room. Her suspicions confirmed, Sybil returns to the lobby and promises to make Basil regret this for the rest of her life. When Basil again attempts to pin the blame on Stubbs, Sybill explodes.
Infuriated that Basil had hired O'Reilly behind her back despite his previous disastrous jobs, Sybil rants at her husband over his habit of hiring O'Reilly purely because he was cheap, or as Basil called him 'cheapish'. At one point, Sybil flings a money box at him, which causes a crash that O'Reilly overhears from the bar and leads him to walk in just as Sybil is vilifying the builder, calling him a half-witted, thick irish joke. Basil quickly tries to pretend that they were talking about another incompetent irish builder, but Sybil makes it very clear who she was talking about. O'Reilly foolishly makes light of Sybil's displeased reaction to the mess he has made, and when she asks him if he thinks she is joking, Basil is horrified when the builder makes the big mistake of smiling at her. Sybil reacts by pulling out an umbrella and pummeling the hapless O'Reilly to the ground. She proceeds to tell O'Reilly that she had seen more intelligent creatures than O'Reilly lying on their backs at the bottom of ponds, and orders him to collect his things and get out. With most of her anger spent, Sybil then calls Stubbs and asks him to come to the hotel in the morning to fix the mess O'Reilly made, then departs to stay at Audrey's for the night. Shaken by the attack, O'Reilly prepares to leave. However, Basil is determined to prove Sybil wrong, and convinces O'Reilly into doing the best day's work he'd ever done.
When Sybil returns the next morning, she finds that O'Reilly has seemingly not only rectified the mess, but also blocked off the drawing room door and installed the kitchen door just as desired. When Mr. Stubbs arrives and considers the work to be a good job, Sybil is faced with an embarrasing situation. Basil revels in having gotten one over on her, but his gloating is to be short lived: when Stubbs questions him over what type of lintel O'Reilly used for the kitchen door frame, Basil states that he used a wooden one. Stubb's reaction quickly changes to alarm. Since O'Reilly has used a wooden lintel for a supporting wall, the hotel's upper floors are in imminent danger of collapse. After informing the shocked Sybil of the crisis, Stubbs quickly races to find a screwjack to prop up the wall before the hotel collapses.
This latest botched job is the final straw for Basil, as the vengeful manager storms out the hotel with Sybil's gnome in hand, intending to carry out his previously mentioned threat. O'Reilly is neither seen nor mentioned again after this episode, so it can be assumed that Basil learned his lesson about the dangers of hiring cheap builders.