You get to keep the car irrespective of whether the company is paying you. An employee maintains all of her contractual rights whilst absent on maternity leave - with the exception of the right to remuneration. Therefore an employer does not bear any legal obligation to pay an employee while they are on maternity leave. An employee in such cases will apply for maternity benefit from the Dept. of Social Welfare.
However, in the particular case of maternity leave, there is a very narrow definition as to what constitutes "remuneration" - under European law, it is limited to the payment of wages or salary. This does not include an entitlement to a company car.
As such, you will be entitled to keep the company car whilst on maternity leave - on the basis that it is not remuneration.
In all other cases of protective leave (e.g adoptive, parental, carers etc.) you will not be entitled to the car as the definition of "remuneration" is much broader, encompassing an entitlement to a company car, and therefore may be suspended for the duration of the protective leave period.