Biggest Irreducible Cat I guess you could call this the next challenge in the &quot;Irreducible series&quot;.

Hello World

Challenge
Create a cat subject to the constraint that it is irreducible.
A cat program C is considered irreducible if there does not exist a cat (in the same programming language as C) that can be constructed by removing characters from C.
For example
console.log(prompt.call());

can have the the bracketed characters removed
console.log(prompt[.call]())[;] -&gt;
console.log(prompt())

And it will still be a cat. Therefore console.log(prompt.call()); is not irreducible.
The winner will be the program with the longest source code.
Extras
A big shout out to @acupoftea for showing that, for any given language, there is actually a maximum score. Otherwise you could construct an infinite set that would break Higman's lemma!.
Jelly -u, \$4\times10^{41}\$ bytes
“...”WẈbØ%ỌV

Except instead of ... its \$133390687877217192365177139021057049493695\$ Ɱ characters
More specifically, a score of \$400172063631651577095531417063171148481103\$
How it works
The standard cat program in Jelly is
ƈȮøL¿

Try it online!
If we convert each of these characters to their Unicode code point we get
[392, 558, 248, 76, 191]

Treat this a base \$4294967296 = 2^{32}\$ number and convert it back to decimal to get \$133390687877217192365177139021057049493695\$
The above program has a string consisting of \$133390687877217192365177139021057049493695\$ Ɱ characters. We then take its length, convert it to base \$4294967296\$, convert back to characters and run as Jelly code.
By forcing Jelly to encode the source as UTF-8 rather than using the Jelly code page, multi byte characters are counted as multiple bytes rather than just 1.
This is 100% irreducible. The cat program it encodes is optimal for Jelly, so there's no way to remove any of the Ɱ characters and still create a cat program, and all of the other characters are necessary to correctly convert the string to a program and execute it. More specifically:

Removing either “ or ” will cause syntax errors
Removing the W will cause Ẉ to return a list of lists of \$1\$s rather than the code points in base \$4294967296\$
Removing the Ẉ will mean the program won't ever convert the string to the code points in base \$4294967296\$
Removing any of bØ% will prevent the base conversion happening
Removing Ọ or V will stop the program from converting to characters and running the program

Furthermore, I believe this is the longest you can get using the method of &quot;long string's length in a high base&quot; in Jelly. The base being used must meet the following criteria:

The number isn't &quot;constructed&quot; via commands to be larger, as these commands can be removed and the long string adjusted for the lower base
The number doesn't have digits in it, as characters can be removed to just isolate the lowest digit and the long string can then be adjusted to match this new base

Ø% is the largest constant Jelly has which meets these criteria at \$2^{32}\$, so, as a higher base leads to a longer string, an answer in Jelly cannot beat this one.