unfortunately jobs are extremely scarce in sligo, it's been hit really hard by the recession, and summer jobs are even harder to get. if someone knows of a job going they are not likely to post it here as they probably have a family member/spouse/close friend who is desperate for work. a lot of people here who do get jobs seem to get them from personal connections but the official job postings would be in the sligo champion and jobs websites like FAS, etc. also, check the websites of the specific company, if they are fairly big they usually have a link where you can apply or a page about any vacancies.
do you have experience pulling pints or waiting tables? because that's a lot more realistic if you want something part time for the summer than anything in business/mechanical, etc. the hotels would be your best bet imo, they get very busy in the summer.
sorry i can't be of more help or have more optimistic info, just seems to be quite dreadful here for jobseekers at the moment.