What age are you? If you have been unemployed for 4 years, then why were you not on a social welfare payment before starting your course?
If you are a full-time student, then there is a general disqualification from Jobseekers during the holiday period. However, if you were 23 or older when you started the course, this disqualification does not apply, although you may still have difficulties persuading them that you are available for full-time work given your intention to start back in college in September. You can of course apply. If you are on Jobseekers for 3 months, then you can apply for Back to Education Allowance (Second Level), although in general this is only for the first year of a course, although you might be able to persuade them to apply the exemption in your circumstances and apply it to the second year.
As far as the depression is concerned, if this preventing you from looking or being available for full-time work, then you will not be eligible for a Jobseekers payment. You may be eligible for Disability Allowance as the previous poster stated, but this will depend on the medical evidence.
If you submit an application for either Jobseekers or Disability, you can in general apply for Supplementary Welfare Allowance from your Community Welfare Officer in your local health clinic and they can pay you until the claim is determined. However Supplementary Welfare Allowance is not available to full-time students either unless you are on the Back to Education Allowance.
It sounds like there is something in the background that is preventing you from accessing social welfare payment (perhaps you have a partner with income, or are under 25 and living with parents???). Anyway, it is quite complicated now that you have entered the education system without going through the social welfare channels, you would be better speaking to an expert on this, perhaps speak to someone in FAS, or the Irish National Organisation of the Unemployed who run information line 01 8560088