In the West of Ireland it is common to hear the expression "she wears the britches" regarding who calls the shots in a marriage.
Also my grandmother (a native Irish speaker) used to say "nothing grows old on a man except his britches". I am still not certain of the meaning of this enigmatic expression.
It's about style/fashion - a man can be buried in the suit he was married in, women's fashion changes more quickly than that. He'll wear out his britches though.
This doesn't have to do with the lesson, but I think the meaning behind what your grammy said might be something along the lines of "If it's good/works like it should, there's no worth in replacing it" (If your pants fit well, why buy a new pair? I guess??). Or something, haha.
My mom used to say "britches" too. As children we used to be impatient about something (usually going swimming or getting candy or something like that) and she'd tell us "hold yer britches" which meant have a little patience, wait just a few moments. I don't know the origin of that saying, but that's what she'd tell us all the time.
I realise this was a long time ago, so apologies if you've found this out already, but "she is wearing the trousers" would be tá sí ag caitheamh an bríste - "she is at wearing the trouser". You use the verbal noun form of "wear". Like English, Irish uses tá... ag [verbal noun] / "... is [verb]ing" more often than the simple verb form, and they have the same slight difference in meaning, so it's important not to mix them up!
I haven't had the pictures yet so "an bríste" is indeed a pair of long trousers, rather than undergarments and it is singular since I have never been sure how you have a pant (Spanish, I'm talking to you)? "An bristí" should be pairs of long trousers, like in one's closet or wardrobe... or in a pile needing to be pressed (which may or may not be where mine are right now)?
We say "un pantalón" in Spanish because it is only ONE piece of clothing. Socks are plural and so are gloves because they are two pieces but "a pant", as we say, is just one piece. Makes much more sense to me and I don't think it's just because I'm a native Spanish speaker. I think it's very logical... xD LOL