This one was suggested by my husband, a keen birdwatcher (see below for why that’s relevant) and adder of troublesome pairs to my list.
Erupt is perhaps the better-known of the two. To erupt is to forcefully throw out rocks, lava, gas and ash, if you’re a volcano. The next meaning is to break out suddenly, usually used of something like a fight, and similarly, you can erupt into laughter, meaning it happens suddenly and forcefully. Finally, a spot or rash erupts when it appears suddenly on the skin, and a tooth erupts through a gum when it grows in your mouth and becomes visible.
To irrupt, also a verb, means to burst into somewhere, to enter suddenly or even forcibly. The kind of thing people do when they break down a door. Interestingly, I’ve seen people being described as “erupting” in this sense, but let’s use irrupt here if we can, to preserve those two senses, yes? The second (and husband-relevant) meaning is to migrate into an area in large – that’s abnormally large – numbers, and it’s especially used of birds. So when an awful lot of waxwings descended on some trees with berries in a Birmingham car park, that was them irrupting. Or an irruption.
You can find more troublesome pairs here, and here’s the index to them all!
LibroEditing proofreading, editing, transcription, localisation 
Rebecca Foster
August 16, 2017 at 11:01 am
I always presumed “irrupt” was just an archaic spelling! My husband is a birdwatcher too, so I liked your example. We’ve gone looking for waxwing invasions many a time (and occasionally found them).
LikeLiked by 1 person
Liz Dexter
August 16, 2017 at 11:17 am
It’s amazing what you find out when you stop to look these things up, isn’t it! I’ve not seen a waxwing, but my husband saw a good one when I’d chosen to go for a long run rather than search them out with him!
LikeLike
Don Massenzio
August 17, 2017 at 2:52 pm
Reblogged this on Author Don Massenzio and commented:
Here is a helpful tip from the Libro Editing blog on the uses of irrup and erupt.
LikeLike
patriciaruthsusan
August 17, 2017 at 3:26 pm
Interesting post. Thanks for the information. 🙂 — Suzanne
LikeLiked by 1 person
Liz Dexter
August 18, 2017 at 6:18 am
You’re welcome, glad to provide interesting information!
LikeLiked by 1 person
suzannebowditch
August 18, 2017 at 9:12 am
Reblogged this on suzannebowditch and commented:
Really helpful grammar tips here for writers
LikeLike