In manuscripts—and even in print—I frequently see the following:
Let’s go to the Molloy’s house.
Grammatically, this means:
Let’s go to the house of the Molloy.
Now, perhaps there is a big burly guy out there who is referred to as ‘the Molloy’, as in ‘Give that burrito to the Molloy before he chews someone’s arm.’ In that case, the above would be correct. But what the writer usually means is:
Let’s go to the Molloys’ house.
Let’s go to the house of the Molloys. [a couple, or a family, or the three banjo-playing Molloy sisters]
If, on the other hand, the writer is referring to a particular Molloy:
Let’s go to the house of Molloy. [e.g. Joe Molloy]
then it would be:
Let’s go to Molloy’s house. [singular Molloy; no definite article]
Again, it comes down to singular versus plural: one Molloy or a bunch of them!