Planning will be the hardest part to put an exact time on. In our case it took about seven months from application date to granting of planning. This was due to a raft of further information requests on items which had already been agreed at pre-planning meetings. These are not reasonable people.
From the build point of view it will indeed depend on whether you get a main contractor or go down the direct labour route. I went the direct labour route myself and am happy I did. We're on the last stretch and all going to plan will be in on the 18 month mark. Looking back it could have been done quicker but we weren't in that much of a panic. Direct labour does require quite a lot of effort, supervision, organisation etc but it does allow you to make fairly significant savings and I think if you can afford to dedicate the time and effort to it, you have more control over the final finish.
That said if you cant afford to dedicate pretty much all of your free time for a couple of years to it, get a main contractor.
Good quality builders around the south were quoting around €70/sq.ft last year for a builders finish. But things are so tight nowadays you would really want to pay very close attention to what is and is not included in the quotes. I will be fully finished and furnished for about €65/sq.ft (thats oak kitchen, solid oak floors, marble and travertine tiles, solid oak stairs, limestone cills, 120m2 limestone stonework, triple glazed windows, aluminium rads etc.) So €100 /sq.ft even in Dublin should get you a really nice finish whatever way you go.
If you already have a site and €75k in savings you're not exactly sub-prime. Despite what it may seem like from listening to the news, the banks are still giving mortgages. They are just way more cautious. Speak to a mortgage advisor, preferably one who is not affiliated with any bank in particular. They will go through the process with you.
Best of luck with what will be one of the most life consuming, frustrating,tiring, exciting, enjoyable and ultimately rewarding experiences of you life.