Well... you can try "
http://www.irishpage.com/irishpeople/". This page has a lot of information (128 lessons) on the Irish language. I think it's the best one.
And you can also go to "
http://www.smo.uhi.ac.uk/gaeilge/gramadach/canuinti.html" and have an idea about some differences between the most important dialects of Ireland and Northern Ireland.
The page "
http://www.taic.btinternet.co.uk/" has information on Scottish Gáidhlig, and that's a pretty good one.
At "
http://www.ling.uni-potsdam.de/~green/gaeilge/gaeilge.htm" you can find some good material on Irish (the page is originally in German, so look for the English translations of the archives if German is not your best).
In "
http://www.irishgaelictranslator.com/" there is a forum with people interested in Irish and some lessons on the grammar which are given by them.
In "
http://www.maths.tcd.ie/gaeilge/" there are eight lessons that can give you an idea of the Irish grammar. You can do this one first.
I particularly think that you have not only to enter these sites, but to search more sites (especially on pronounce... you can't trust the pronounce rules of most sites in Internet), so that you can have an image of Irish.
Of course none of these sites is better than a teacher or an Irish course, but if you prefer, you can enter these...
