I don't understand these tire sizes.

I found an old 1978 BeeKay road bike in my girlfriends shed, it was her Dads from when he lived in Montreal. It's all Japanese parts assembled in Canada. I took it out of the shed, pumped up the tires, fiddled with the brakes a little and it works perfect. However the tires are very old and have cracks in the side walls. The rear tire is made by Norco and says to inflate to 90psi where as the front tire is made by Dunlop and states 70psi is required. Now I know that won't effect much of anything but while I was looking for new tires I found three different sizes for road bikes. 700 x 23c, 700 x 25c, and 700 x 28c but the tires on the bike say 27 x 1 1/4. I don't know which tires I'm buying for it because I couldn't find anything telling me which one is for 27 x 1 1/4? Any help would be great!