Irish-American tendency toward Democrats

Irish Americans have shown themselves as pro-Democrats through 150 years living, formally as an American ethnic group, in U.S. You can find it out by focusing on results of Americans' presidential elections along one and half century ago.
In retrospect to twentieth century,the trend had been more shining ,for they even developed their political participation to high status such as presidential candidacy in 1920s and 1960s.Two politicians with Irish ancestry imposed themselves as democratic candidates and participated in elections of 1928 and 1961.In 1928 the Irish Al Smith was defeated by Herbert Hoover, but the latter, John F. Kennedy, could defeat his rival, Richard M. Nixon, in 1960 and exposed himself as the first Irish catholic president. He could take the views by his reformist plans and ideas. The views which doubted his deserve for presidency for his being youth, naïve and Catholic.
Voting for presidents, they have attempted to send a Democratic president to White House rather than a Republican. Since 1830s to 1960s, Irish Catholics voted for democrats up to 80-95%. As getting more far than 1960s, approaching now, this trend gets more moderate. Based on census in 2004, in presidential election held this year Irish Catholics kept the middle and ran for 50-50 %.
Among Irish Catholic politicians, also, there are more toward Democrats, even though some are of republican leaders. Some American presidents have claimed to have Irish ancestry such as Ronald Reagan (president of U.S from 1980-88).Nowadays, the pro-life silent majority in Democratic Party are Irish Catholics. According to recent census,"8 out of 28 American presidents have had Irish ancestry."

Why?
Maybe it can be considered that Liberalism and Democrats' supports on emigrants and ethnic groups' rights have been the main influencing factors on Irish Catholics trend in not so far era. The factors which most ethic groups in America have same focus on them.
A