In the Gaeltacht, would I visit 'an stáisiún gáis' or 'an stáisiún peitril'?
"An stáisiún peitril." The term "gasoline" is rarely, if ever, used in Ireland, though there is an Irish translation for it ("gásailín"). "Gas" therefore refers to the state of matter, and most often to natural gas. This is true in Irish as well as in English[*].
"Stáisiún peitril" is the only translation offered for "gas station" in the NEID: http://www.focloir.ie/en/dictionary/ei/gas+station
[*] Because somebody's bound to mention it, the Irish word "gás" apparently can also refer to paraffin oil, but I would be surprised if that usage were still current: http://www.teanglann.ie/ga/fgb/g%c3%a1s