People can be ethnically Jewish or religiously Jewish and they are separate identities. Historically, religiously jewish people tended to only marry other religiously jewish people, leading to the formation of a jewish ethnicity over time. For many, these identities are closely intertwined, for others they have both but view them separately. And for many others still, they only fit into one category or the other.

Irish, in contrast, is only an ethnicity but not a religion. (Unless you count certain sects of Celtic Paganism, but that's usually not what people mean)

If one parent is predominantly of Jewish heritage and the other of Irish heritage, then their child might identify as half-jewish-half-irish.

Genetically speaking, they are likely less than 50% of each because that would imply that each parent was completely and totally 100% their respective ethnicity genetically, which is (if possible) very very unlikely and realistically not 100% strictly defined.

People like to categorize things, including categories. For some, a part of their identity is based on the ethnic categories they fit themselves into, and some group these categories under one subsection of their identity, and assign weights to the different components of that category.

I love the funny things our pattern seeking brains do in order to quantify the unquantifiable and to better establish a sense of belonging and self in this amorphous and crazy society we're all a part of. What's really great is that none of what I've said is even universally true. It's just (from my observation) the most common way I've seen all these categories combined. If you disagree, you're completely free to do so, and neither of us are wrong until we start using numbers and statistics in our argument