Sometimes peoples salary may not reflect their responsibilities accurately. This may happen in the case of new graduates/school leavers. They may be happy to take on a lot of responsibility for low pay, just to get the experience.
However, if these people are moving jobs, then the new employer should pay the 'going rate' for their experience. Some new employers will still only pay 2k more than the employee is currently getting.
Also people who get made redundant might take a lower paying job just to stay in the workforce. If they are changing jobs, surely they are entitled to a salary to reflect their previous experience/qualifications. The employer should not have the attitude that X is only earning 22K, so they will be happy with 24K, even though they could have 20 yrs experience at a senior level. Some employers actually use this just to get someone to work for low pay.
Also, changing jobs can be a bit of a gamble. A lot of things could go wrong. If you are found to be unsuitable, you could be let go, and then you would have no job!! You are taking a chance in that the new working conditions could be less desirable, the people could be difficult to get on with etc. Add all this to the effort of starting a new job, putting in extra hours to learn this job, and an extra E20 net a week does not make this worthwhile.
I feel that I would need to get about 3 to 4 K extra in order to change jobs. By the time you pay tax, you would need to take home E40 net extra.
The way to get around this with prospective employers is to cost out your benefits, eg free canteen, VHI etc. When asked what salary you are on, you could add in these benefits and give a range e.g 30 to 35K for your job. Then you are not caught out in a lie. If there are any queries about your P45, you could say that you just got a pay increase in your last job. You could always say that some of the benefits in your last job were non monetary.