The company – Europe's largest airline – made post-tax profits of £1.14 billion ($2.1b) in the six months to September and its chief executive Michael O'Leary earned a pay package of £2.8 million ($5.3m) last year.
Lawyers who assessed the Mail's evidence accused Ryanair of "staggering sharp practice".
A probe has been launched by Parliament's work and pensions and business committees.
The Mail investigation found:
• Ryanair's stewards are charged more than £2,000 for training;
• Recruits pay for uniforms and are charged a fee if they quit;
• Every year, some agency staff must take up to three months of unpaid leave when it is quiet;
• Bosses threaten to relocate them abroad if they do not sell enough during flights;
• They must be available for standby days where they are paid £3.75 per hour – less than half the minimum wage.
Staff issues have led to 700,000 bookings and more than 20,000 flights being cancelled by Ryanair this winter. Its pilots last night suspended a strike over union recognition.
Ryanair has said it will recognise pilots' unions for the first time. But cabin crew say they are still threatened with the sack if they strike.
The Mail reporter was taken on as a trainee by Crewlink, one of two firms that hire for the airline. Last year, it had more than 3,000 on Ryanair's books. Overall, the airline has about 8,000 stewards.
The reporter was told she would earn a 'premium' hourly rate of £14.43 ($27.60), plus commission of 10 per cent of in-flight sales. But the hourly rate would apply only to "flight time", from when a plane leaves the blocks till it is parked at its destination.
Recruits were told the unpaid work would consist of at least 45 minutes before the day's first flight, time between flights, and half an hour after the final flight. Instructor Dorota Sowinska said they could work for ten hours in a day but get paid for only five.
Stewards hired directly by Ryanair get a basic gross annual salary of £9,616 ($18,396) with a lower hourly flight time rate.
They have access to a company pension scheme and sick pay. Agency recruits can apply to be full-time staff after a year but many are kept on agency contracts for far longer.
Many crew said they estimated only about 20 per cent of colleagues were on direct contracts.
The Mail's reporter was told she could top up earnings through sales. Until 2015, Ryanair stewards earned commission on what they sold as a group.
Now they earn on individual sales – and are given strict targets and a raft of hard selling techniques.
Agency cabin crew face costs of least £2,150 ($4,113) for training and £25 ($47.82) per month for uniform in the first year.
They are paid a £1,000 ($1,913) allowance in the first year, but this can be claimed back if they quit in this time.
A 2017 contract seen by the Mail states that a steward would have a £175 ($334) "administration cost" taken from his salary if he left in the first 15 months.
Of seven airlines contacted, Ryanair is the only one that hires through third-party agencies which offer no basic salary.
EasyJet, British Airways, Jet2, Virgin Atlantic, FlyBe and Lufthansa hire directly and pay staff basic annual salaries from £14,069 ($29,914). All offer free training, except Jet2 which charges £700 ($1,339).
The Mail's reporter was told she must be available for standby shifts – at home, when crew are not paid but have to stay an hour from the airport; or "airport standby", when they must be on site in uniform and can be made to clean and sell tickets.
For these eight-hour shifts, they are paid £30 ($57.39), or £3.75 ($7.17) per hour. Ryanair said this is lawful as total pay is above minimum wage when flight time and sales commission is included.
Edward Cooper, of law firm Slater and Gordon, said charging for leaving a job is "staggering and warrants further investigation".
Lawyer Nicholas Evans, of Fletcher Day, said Ryanair could be breaking the law if it does not record stewards' full work hours, adding: "The evidence indicates there's a lot of sharp employment practices going on."
Frank Field, of the work and pensions committee, said: "The dice are loaded in favour of mega-profitable companies who are willing to shamelessly exploit workers to obtain a competitive advantage … we will be investigating these allegations further."
Rachel Reeves, of the business, committee, said: "These allegations suggest a company falling well short of its duty to staff." The transport committee's Lilian Greenwood said: "Low prices can never come at the expense of fair, safe, legal treatment for staff."
Ryanair denied any wrongdoing. It said agency cabin crew's hourly wage for flight time covers all duty time, including on the ground, and full work hours are recorded as the law requires. It said minimum wage legislation does not cover standby duties.
The firm said it is entitled to put small numbers of personnel on unpaid leave in quieter periods and average pay for crew, including agency workers but not supervisors, is £21,140 ($40,443). It said recruits who stay for a year get an "annual uniform allowance" of £396 ($757).
Crewlink declined to comment.
"Flight delayed? The rules are you're only paid for hours you're in the air"
In a classroom on an abandoned airbase in rural Germany, Dorota Sowinska addresses 35 of Ryanair's latest recruits.
The air stewardess, who is wearing the budget airline's trademark blue and yellow uniform, is standing in front of a large white flipchart. At the top, she has written the words "FLIGHT TIME".
"You are only paid for the hours in the air," she announces. "If you have a delay and you are staying somewhere, you are not getting paid for the time on the ground."
Miss Sowinska adds that in a ten-hour shift, the Ryanair stewards may end up getting paid for only five. This is because the work they do when not flying – including cleaning the plane, checking for explosives, boarding the passengers and filling out paperwork – will be totally unpaid.
The significance of this appears to be lost on many of the novices, who earnestly note down the instructor's words. Some of them are just 18.
Also in the room is an undercover Daily Mail reporter, posing as a recruit. She presses Miss Sowinska – why aren't they paid for so many hours of work?
The Ryanair trainer offers no explanation, simply responding: "It's the rule."
It is early November and the class of 35 has been flown to the six-week training camp by Crewlink, a third-party Irish firm used by Ryanair to hire cabin crew. In total, there are almost 300 recruits, split into eight classes.
The course is a fast-track to a career in the skies. Within hours of completing it, those who pass are flown straight to one of 84 bases across Europe. At the course attended by the Mail, trainees come from Italy, Spain, Portugal, Slovakia and the UK – and are due to be posted to British airports.
They soon discover that Ryanair's notorious cost-cutting extends to the conditions for recruits on the course.
The Hahn Training Centre in western Germany is on an isolated former airbase, with dozens of derelict barracks. The main training building consists of little more than threadbare classrooms. In the thick fog, one student describes it as looking 'like a horror film'.
Between classes, trainees queue in the cold to buy food from a bread van that visits once a day. Inside, there is one vending machine. There is also a shuttle bus so they can get food from an airport terminal a mile away.
The closest supermarket is Lidl, a 40-minute walk away along a dual carriageway. To avoid high prices at the airport, most go to Lidl in the dark after lessons.
Their accommodation is a former hotel with a boarded-up reception, dank laundry room and one kitchen for up to 230 people. The showers are often cold.
The recruits begin each day standing in front of the class while their appearance is critiqued.
Women's hair must be worn in a French roll, or a bun up to 6in in diameter. Nails are checked for chips. Lipstick must match nail colour exactly. For men, facial hair must be no longer than 12mm.
If they get the job, the recruits face "grooming discrepancy" warnings detailing any failures, such as "messy hair". Getting too many of these leads to dismissal.
The course is very thorough. Hours are spent on safety procedures, training on dealing with bombs, fire-fighting and first aid.
Students are failed if they do not achieve 100 per cent attendance.
For every minute a student is late to a lesson, ten minutes are added to the end of the working day for the entire class. They are often forced to stay at the centre many hours into the night.
While the job is advertised as offering "great earnings potential" and "great benefits", the trainees face repeated demands for cash.
As well as more than £2,000 ($3,826) in training fees, often deducted gradually from their wages, they pay a non-refundable £435 ($832) registration fee and £625 ($1,195) for their bed-only accommodation during training.
They will later be billed for an airport ID and uniform, and are charged a fee if they leave the job within the first 15 months. Crewlink hires hundreds of recruits a year for Ryanair.
Some 90 per cent pass the training, but up to 30 per cent of them have their contracts terminated within the first 12 probationary months. However, they are still legally contracted to pay back their course fees and any allowances paid to them.
Asked why they persevere with the course despite the grim conditions, one 20-year-old Slovakian said: "We've already paid enough to be here, and we have to pay it back if we quit. Besides … it is better money than in my country."
A British girl added: 'I've been ringing [my parents] complaining and they said, 'By the sounds of it you're not having a very good time for something you want to do'.'
Others spoke of how they planned to leave Ryanair after the first year and find work with another airline. But many said quitting was too expensive.
The airline said conditions at the training centre were "basic but acceptable" and the school was certified by the Irish Aviation Authorities for holding training courses.
It said it provided €16 ($26.95) per night accommodation but recruits could choose to stay elsewhere. A spokesman said its grooming rules were "fair and balanced and typical of all airlines".
