You have made a wise decision....even though your boyfriend might not like it!
Most people who travel to Australia do it solo so you are definitely not on your own! You will meet people on the first day, probably already at the airport in Northern Ireland who are on exactly the same trip as you! And if you are open to new people, within a week you will be partying like a rockstar with a bunch of new friends....guaranteed!
Now, cheap accommodation is always the biggest worry when getting to Oz but most people find a solution in the first couple of weeks. August is not the busiest time in Sydney because it is their winter (although still very moderate) so you will still find it reasonably easy to get a place I would think. I would recommend Coogee, Bondi, Bronte, Manly as beaches with a good vibe, even though they are run over by backpackers and students (this isn't necessarily a bad thing.... I loved living in Cooge for 9 months!). Finding a cheap place can be tough here though and get ready to share a room with at least one other person if you don't want to pay more than 110 AUD a week in Sydney (pretty much everywhere). There is a well known street called Hall Street in Bondi where there is a window plastered with accommodation offerings...I think it is 5 Hall Street. In Coogee there was a place next to the Mac Donalds which was run by a really nice guy and he rented out some of his houses and appartments to backpackers. Other nice places in Sydney to live are Glebe (heaps of students here) and Darlinghurst for example. These are more city central areas though and 95% of the backpackers prefer the beaches. Check out the Sydney Morning herald on Saturday for most accommodation listings. Wednesday is the other day I think they have accommodation listed. Besides that, hostels are a good place to find accommodation as most have notice boards that are readily used by other travellers. And don't forget the good old phone book where under real estate agents you will be able to find a grand selection of agents all over the city. The only problem with these is that they often require you to sign a lease for at least 6 months (usually a year) and most backpackers tend not to stick around for that length of time.....
Regarding work you should probably be ok as well at that time of year. I would stay away from places that charge you money and say they will get you work in x amount of days. They usually do offer you something but it could only be for 8 hours and then they have lived up to their end of the deal in their eyes while you are at least 200 bucks lighter. And these so called 'backpacker agencies' usually get the work from other employment agencies anyway. All other employment agencies don't charge anything and will have jobs going here and there for backpackers. Just make sure all of them (or a lot of them) have your resume so you get the calls when they get the work. Most people just tend to find something more permanent on their own though...bars, cafes, restaurants, hostels etc. are your best bet here. Oh, and call center work is always available!
Hope that helps....
Cheers,
