This is the case across a range of Trinity departments; I myself have a PhD offer from the history department, and I am currently finishing up an undergrad degree.
Obvious advantages are that you can jump straight into research without going through the Master's stage, especially if you know what topics you want to study. Bear in mind though, that Trinity PhDs are usually four years, compared with, for example, in England a lot of funding is only for 3 years for PhD.
Trinity students study for four years for their undergraduate degrees, even those doing arts-type subjects. It is the opinion of many (though not all, it must be acknowledged) that Trinity students do not always need to study for an intermediary degree (i.e. Master's) between undergrad and PhD, especially if your undergrad subject included a research dissertation/project. Of course, some would benefit from this, especially with regard to developing some research-related skills, but it really is a personal thing. For instance, my prospective PhD supervisor has spent the year trying to convince me not to bother with a Master's, because I already have the interests and skills for research; that is not necessarily my opinion too though, and I have applied for a few Master's programmes as well, and am now weighing up my options.
In the case of Trinity's PolSci programme, a lot of the students on it do have some kind of Master's degree. A good few come through the ranks of the Department's own master's programmes (they actively recruit these students), and they also receive a lot of American students, who have often done some kind of Master's in the UK. Be sure to contact your prospective supervisor beforehand and really nail the research proposal!