Louis Eppolito and Steven Caracappa served in the NYPD for a combined 44 years, but were also on the payroll of the Luchese crime family under boss Anthony 'Gaspipe' Casso. An extreme example of moonlighting perhaps, but when does taking on a second job become unethical or a sackable offence?

Moonlighting is only a problem if the employee burns the candle at both ends, according to Ronnie Neville, senior associate at Mason Hayes & Curran Solicitors.

"There are a lot of issues that arise with moonlighting, " he says. "For example, if the employee comes in with dreary eyes or productivity begins to suffer, then you're on a disciplinary road. It's not specifically because the person is moonlighting, but because it is having a detrimental effect on his or her work."

Moonlighting can have a detrimental effect on both the health and productivity of an employee. Very often the primary job will demand extra hours and a hectic schedule. If a person takes on consulting clients, for example, they too may put enormous pressure on the individual. Some practical advice would be to make sure the night job doesn't interfere with the day job.

There are two types of moonlighting. The first is where someone wants a second job that is completely different to their main job. For example, an office worker may moonlight in a band at the weekends. The second type of moonlighting is when a person decides to do the same type of work at home as in the office, but take on their own clients. The conflict of interest arises when you're working for a direct competitor.

"If someone working for an employer knows that they are going to leave in a month's time, poaches clients, and lets those clients know they are on the market, it is a breach of the duty of trust and fidelity, " says Neville.

"If the employee is working in a capacity or function that is not in the best interest of the company, again there may be a breach of trust and fidelity.

The employer could take proceedings against that person."

Another issue for employers is the fact that the onus is on them to ensure their employees are not working too many hours, under the Organisation of Work Time Act, 1997.

"Employers may want to protect themselves by having a clause to the effect that the employee can only work so many hours, " says Neville.

"They may require you, the employee, to reduce your hours, because otherwise you are exposing them as the primary employer."

While pursuing a second career that is completely different to your first may not appear to cause a problem at first glance, it could do so in some professions, says Neville.

For example, a teacher's moonlighting activities may not be setting a good example for their students.

"Even if someone isn't breaching the Work Time Act, has a constitutional right to work and doesn't have a clause in his or her contract, is the employer entitled to make him or her give up the other job?

The initial answer is no for a regular job, but there is an implied term in a contract that you will act in the best interest of the company."

Neville says that, while it is unusual to have a clause of exclusivity for a regular worker, it is the norm for senior people and professionals to have one in their contracts.

Bringing up your intention to moonlight at the initial stages of your employment may be one way to avoid getting into legal wrangles later on. Letting a potential employer know what kind of consulting work you intend to do and explaining that you won't compete directly with them will help reassure them that your day job will come first. However, this could affect your chances of promotion in your new job.

If a person's moonlighting has nothing to do with his or her chosen career and is therefore not in direct conflict with it, an employer cannot fire that person for moonlighting, says Neville.

"If an employer plays hard ball and if there's no objective reason or grounds to criticise the employee for poor productivity or the manner in which he or she does the job, then arguably you've got a breach of contract, " he adds.