Very readable accounts of Irish etymology -- although very limited in scope -- can be found at these two sites by Dennis King:
Stair an Fhocail
http://www.smo.uhi.ac.uk/gaeilge/donncha/stair-an-fhocail/ Focal an Lae
http://www.smo.uhi.ac.uk/gaeilge/donncha/focal/ And you can also visit Dennis's blog where, just today, he's been discussing the etymology of "slaghdán":
http://nimill.blogspot.com/ What this man doesn't know about the history of Irish probably cannot be known.