Under section 98 of Employment Rights Act 1996, in determining whether the dismissal of an employee is just or unjust, the employers have the obligations to prove that there were solid reasons for dismissal.
Failure to prove -> unfair dismissal
Qualifying period increases
These reasons are:
i) Relates to capability or qualifications of the employee for performing work of the kind which he was employed by the employer to do,
Inherent incapability -inaptitude ~in Blackman v Post Office  ICR 151 the employee repeatedly failed a test, was neglectful (but, in Davidson -v- Kent Meters 1975 there was an expectation that the employer, normally, should train or give another job).
Blackman v Post office 
They must be ‘for performing work of the kind which [the employee] was employed by the employer to do’. [15.41] Self-evidently they cover formal qualifications, technical or professional. They relate to an employee’s aptitude and ability. Thus, an employer’s aptitude tests can constitute ‘qualifications’ Blackman v The Post Office  IRLR 46 (NIRC). Dismissal for lack of qualifications essential for the job not unfair (http://www.bloomsburyprofessionalonline.com/view/dismissal-law-ireland/DL-ch15-UID16.xml)
What is capability?
Capability means capability assessed by reference to skill, aptitude, health or any other physical or mental quality (section 98(3)(a)). (SAU4)
Generally, it will be impossible for an employer to dismiss fairly for an act of poor performance without having first warned the employee that an improvement is required, perhaps by way of a performance review. Employees should be warned of the consequences of there being no improvement and given a reasonable opportunity to improve. In the case of Davison v Kent Meters Ltd1, Mrs Davison was given one month's notice after discovery that, out of 500 components assembled by her, some 471 were faulty due to the fact that she had assembled them incorrectly. In her defence, she stated that she had assembled them as instructed by the foreman. The foreman denied this and she was dismissed. The tribunal held that it was unreasonable to dismiss her without having warned her formally that, unless her performance improved, she would be dismissed. It further found that the foreman should have supervised her properly to correct the way in which she was operating. Therefore, employers should consider whether they have been partly at fault in failing to supervise or train adequately.2 1  IRLR 145.
However, major matter
In the following example of a competence matter, it was found to be fair to dismiss in the first instance.
Alidair v Taylor 
T was a pilot who crash-landed a passenger plane in good flying conditions. Nobody was hurt, but the plane was badly damaged. After a full investigation and disciplinary hearing, T was dismissed for gross incompetence. He complained that as he had an unblemished record he should not have been dismissed for a first offence. The Court of Appeal upheld the decision to dismiss, saying that in some professions the degree of skill needed was so high and the likely consequences of deviation from that level of skill potentially so serious that it would be fair to dismiss in a first instance. Source:
A commercial pilot, T, made a faulty landing while flying 77 passengers in reasonable weather conditions. No one was hurt, but the aircraft sustained...