One of the major legacies of nearly five centuries of Ottoman rule was the conversion of up to 70 percent of the Albanian population to Islam. Therefore, at independence the country emerged as a predominantly Muslim nation, the only Islamic state in Europe. No census taken by the communist regime after it assumed power in 1944 indicated the religious affiliations of the people.
It has been estimated that of a total population of 1,180,500 at the end of World War II, about 826,000 were Muslims, 212,500 were Orthodox, and 142,000 were Roman Catholics. The Muslims were divided into two groups: about 600,000 adherents of the Sunni branch and more than 220,000 followers of a dervish order known as Bektashi, which was an offshoot of the Shia branch. Bektashism was regarded as a tolerant Muslim sect that also incorporated elements of paganism and Christianity.
http://www.country-studies.com/albania/religion.html
I read somewhere that a lot of the "conversions" were actually imported muslims from the Caucasus area.