"Seall! Aon chù!"
Translation:Look! One dog!
lenition is a fancy word for aspiration. Essentially, it means you soften the initial consonant sound. The sound is similar to what it would be if you tried to pronounce the word whilst out of breath. It often happens between two words that are linked together somehow.
In counting, both the numbers one and two cause lenition to whatever noun is following them, but the numbers three onwards don't. To be honest I have no idea why. They just do. I imagine it is somehow connected to the fact that the numbers one and two also use the singular noun and the number three onwards uses plural, but that's just a guess.
So aon chù, dà chù, but trì coin.
Or aspiration is an old-fashioned word for 'lenition'. It is now considered completely obsolete and should not be used. Worse, it is also a synonym for what is now called the spirant mutation in Welsh, and that is a completely different mutation, even though there is a mixed mutation and there are some phonological similarities. So the word aspiration is best avoided at all costs.