Republicans are fine with US mulitnationals locating in Ireland.
Obama is a protectionist who is proposing measures to prevent US companies locating overseas.
Both McCain and Palin have genuine Irish roots - McCain's family is from Donegal. Palin has very close Irish ancestory & had an Irish-American catholic upbringing. Her mother's family - the Sheerans - are Irish emigrants.
All Obama can conjure up is a remote possibility of some very very distant relative with no concrete proof.
In general terms, Republican Party policies (with the exception of foreign policy) are almost identical to the policies of the main Irish parties. The Republicans are essentially a hybrid of FF, FG & PD in outlook and so would appeal to the majority of the Irish electorate on most domestic issues.
In general terms, the Democrat Party are more similar to the UK Labour Party - to the left of centre, though Obama personally is more in the old Labour pro-union, pro-protectionist mold. Democrat views on a lot of issues would generally be at odds with the majority of the Irish electorate e.g. abortion.
Traditional "Irish" support for the Democrat Party is more to do with them being more favourable to immigrants than the Republicans - the support was due to 1 issue which was very very important to Irish people who immigrated in the later half of the 20th century.