The following references are mostly specific to your question:

**[1]** Adrien-Marie Legendre, **Éléments de Géométrie, avec des notes**, Firmin Didot (Paris), 1794, xii + 334 pages.

A proof of the irrationality of ${\pi}^2$ by the use of continued fractions is given on p. 304.

**[2]** James Whitbread Lee Glaisher, *On Lambert’s proof of the irrationality of* $\pi,$ *and on the irrationality of certain other quantities*, pp. 12-16 in Notices and Abstracts of Miscellaneous Communications to the Sections, **Report of the Forty-First Meeting of the British Association for the Advancement of Science** (August 1871, Edinburgh), John Murray (London), 1872.

The paper is on “.pdf pages” 341-345 of this google books item. The top third of p. 14 discusses Legendre’s proof that ${\pi}^2$ is irrational.

**[3]** Charles Hermite, *Extrait d'une lettre de Mr. Ch. Hermite à Mr. Borchardt* [Extract of a letter of Mr. Ch. Hermite to Mr. Borchardt], **Journal für die reine und angewandte Mathematik** 76 (1873), pp. 342-344.

Hermite shows ${\pi}^2$ is irrational by a method that avoids the use of continued fractions --- a method that very soon afterwards led to his proof that $e$ is transcendental.

**[4]** Alfred Pringsheim, *Ueber die ersten beweise der irrationalität von* $e$ *und* $\pi$ [On the first proof of irrationality of $e$ and $\pi$], **Sitzungsberichte der Mathematisch-Physikalischen Classe der K.B. Akademie der Wissenschaften zu München** 28 (1898), 325-337.

If you can read German (I can’t), I believe this could be of use. Lambert’s proof that ${\pi}^2$ is irrational is mentioned on p. 326 (line 9).

**[5]** Sylvain Wachs, *Contribution à l'étude de l'irrationalité de certains nombres* [Contribution to the study of the irrationality of certain numbers], **Bulletin des Sciences Mathématiques** (2) 73 (1949), pp. 77-95.

**From MR0033299 (11,418a) review by Jan Popkin:** By considering a larger class of similar integrals the author intends to obtain more general results. He gives applications by showing in this manner the irrationality of such numbers as ${\pi}^2,$ $\log A$ $(A \neq 1),$ $e^A,$ where $A$ denotes a positive integer. [The paper contains some misprints and other mistakes; the most serious one at the end of § 6, where the quantity $M$ introduced depends on $n.$ In view of various papers giving generalizations of Niven's method it is perhaps of interest to remark that there exists a close connection between this method and the classical proofs for the irrationality of $\pi$ and ${\pi}^2$ of Lambert, Hermite and others. Take for instance the integral $[\cdots]$

**[6]** Yosikazu Iwamoto, *A proof that* ${\pi}^2$ *is irrational*, **Journal of the Osaka Institute of Science and Technology. Part I: Mathematics and Physics** 1 (1949), pp. 147-148.

**[7]** Ivan Morton Niven, **Irrational Numbers**, The Carus Mathematical Monographs #11, Mathematical Association of America, 1956, xii + 164 pages.

The *Alternative proof* of Corollary 2.6 on pp. 19-21 gives a proof that ${\pi}^2$ is irrational.

**[8]** Kustaa Aadolf Inkeri, *The irrationality of* ${\pi}^2$, **Nordisk Matematisk Tidskrift** 8 #1 (1960), pp. 11-16 and 63.

**[9]** John Douglas Dixon, $\pi$ *is not algebraic of degree one or two*, **American Mathematical Monthly** 69 #7 (August-September 1962), p. 636.

Regarding this result, see Proof of $\pi$ not being a quadratic irrational number.

**[10]** Theodor Estermann, *A theorem implying the irrationality of* ${\pi}^2$, **Journal of the London Mathematical Society** (1) 41 #3 (1966), 415-416.

**[11]** Jaroslav Hančl, *A simple proof of the irrationality of* ${\pi}^4$, **American Mathematical Monthly** 93 #5 (May 1986), pp. 374-375.

**[12]** Darrell Desbrow, *On the irrationality of* ${\pi}^2$, **American Mathematical Monthly** 97 #10 (December 1990), pp. 903-906.

**[13]** Michael David Spivak, **Calculus**, 3rd edition, Publish or Perish, 1994, xiv + 670 pages.

In Chapter 16, Theorem 1 (stated and proved on pp. 323-324) is the irrationality of ${\pi}^2.$ This same result probably appears in either or both earlier editions (1967, 1980), but I have not verified this.

**[14]** Miklós Laczkovich, *On Lambert's proof of the irrationality of* $\pi$, **American Mathematical Monthly** 104 #5 (May 1997), pp. 439-443.

**Corollary 2 at the top of p. 441:** “${\pi}^2$ is irrational.”

**[15]** Pierre Eymard and Jean-Pierre Lafon, **The Number** $\pi$, translated by Stephen Stewart Wilson, American Mathematical Society, 2004, x + 322 pages.

Section 4.2.3 on pp. 136-137 is titled “Niven’s proof of the irrationality of ${\pi}^2$”.

**[16]** Paul Joel Nahin, **Dr. Euler’s Fabulous Formula**, Princeton University Press, 2006, xxii + 380 pages.

*Chapter 3: The Irrationality of* ${\pi}^2$ (pp. 92-113) gives a very detailed presentation of the proof in Carl Ludwig Siegel’s book **Transcendental Numbers**.

**[17]** Li Zhou and Lubomir Markov, *Recurrent proofs of the irrationality of certain trigonometric values*, **American Mathematical Monthly** 117 #4 (April 2010), 360-362.

**(2nd sentence of the paper)** We also discuss applications of our technique to simpler irrationality proofs such as those for $\pi,$ ${\pi}^2,$ and certain values of exponential and hyperbolic functions.

**[18]** Timothy W. Jones, *Discovering and Proving that* $\pi$ *is irrational*, **American Mathematical Monthly** 117 #6 (June-July 2010), pp. 553-557.

Niven’s proof of the irrationality of ${\pi}^2$ is discussed on p. 556.

**[19]** Timothy W. Jones, *The powers of* $\pi$ *are irrational*, viXra:1102.0058, 19 October 2010, 17 pages.

**[20]** Jürgen Müller and Tom Müller, *Niven’s irrationality method revisited*, manuscript, undated, 3 pages.

**(3rd paragraph of the paper, on p. 1)** In this note we take a new look at the classic analytic irrationality proofs for ${\pi}^2$ and the integer powers of $e,$ showing that the required approximation polynomials are generated by one single integral expression. Our approach makes it obvious how the polynomials come into existence, why they have integer coefficients and that the irrationality proofs for ${\pi},$ ${\pi}^2$ and $e^k$ are only different special cases derived from the same general formula.

**[21]** lhf, Direct proof that $\pi$ is not constructible, Mathematics Stack Exchange, 30 January 2012.

This might also be of interest.