I think it probably varies a bit. My Mom attended two masses last spring, and I attended one with her. She said that no one was in jeans at the first one, and most of the ladies had skirts on. She actually felt underdressed in her black slacks. At the second mass, there were some extremely casually dressed people, though most folks had nice slacks or skirts.
In '93 we were there for Easter and toted along our best dress-up outfits, went with the B&B hosts to the early Mass and were very much overdressed. Everyone wore very casual clothes. This was at St. Columkille's in Rathcommon, near Rosses Point.
Go casual. The fact that you are attending mass while traveling will score you an indulgence ;)
I've never been to a mass where I felt under dressed in jeans. I've even worn shorts...that was a little under dressed, but I think it was pretty obvious that I was on holiday, and I don't think anyone really cared.
FYI - I can only speak about Catholic services...other faiths may have different expectations.
