The first couple of pages of Masechet Berachot deal largely with the various tannaic opinions as to the earliest time for the recital of Shema at night. The Talmud tries to figure out which of these opinions are in conflict with each other.
The original opinion mentioned in the Mishnah was משעה שהכהנים נכנסים לאכול בתרומתן – when kohanim that had been impure can eat terumah. One Beraita cited contains R. Meir's opinion that the earliest time is משעה שבני אדם נכנסין לאכול פתן בערבי שבתות – when people eat on Friday night. Another Beraita has R. Meir of the opinion that it is משעה שהכהנים טובלין לאכול בתרומתן – when the kohanim perform their ritual ablution.
The Talmud notes that R. Meir seems to have two contradictory opinions here, and it answers that it was in fact a tannaic dispute as to what R. Meir really held. This brings us to Tosafot.
Tosafot is ostensibly explaining why the Talmud did not offer another possible way to avoid the contradiction:
דלא מצי למימר בשעה שבני אדם נכנסין וכהן חד שעורא הוא דאם כן רבי מאיר היינו רבי אחאי לפי' רש"י שפי' שרוב בני אדם נכנסין להסב דהיינו בע"ש
It couldn't say that "the time that people enter" is the same as "kohen" because if that were the case then R. Meir would be the same as R. Achai, according to the explanation of Rashi who explains that [the time of] "most people enter to dine" is on Friday night.
The term "the time that people enter" is R. Meir's view in the first beraita. The term "kohen" is the view in the Mishnah. The term "most people enter to dine" is the view of R. Achai in the last beraita.
In essence, then, Tosafot is saying that R. Meir's view in the first beraita cannot be identical with the view in the Mishnah because then (according to Rashi's explanation) it would be identical with R. Achai's opinion in the last beraita (and R. Meir in that same beraita is ostensibly arguing with R. Achai.
I see several potential issues here:
Why does Tosafot raise the possibility that R. Meir's view in the first beraita is the same as the view in the Mishnah? It is already clear from the Talmud that this cannot be, for R. Meir in that beraita is disputing the Sages who give the same time that the Mishnah does (i.e. "kohen").
How would it help if R. Meir in the first beraita is in agreement with the Mishnah? The current question being dealt with is not how to reconcile R. Meir with the Mishnah, but how to reconcile R. Meir with his own opinion in the later beraita where he gives the time as משעה שהכהנים טובלין לאכול בתרומתן
If, as Rashi explains, R. Achai's opinion of משעה שרוב בני אדם נכנסין להסב is referring to Friday night, then the issue of R. Meir being the same as R. Achai exists even without positing that R. Meir's view is identical to that of the Mishnah.
What I would have expected here would have been for Tosafot to say that you can't say that R. Meir in the first beraita (משעה שבני אדם נכנסין לאכול פתן בערבי שבתות) is identical to R. Meir in the second beraita (משעה שהכהנים טובלין לאכול בתרומתן) because then R. Meir in the second beraita would be saying the same thing as his disputant R. Achai. That would be a simple explanation for why the Talmud had to resort to saying that there was a dispute about R. Meir's opinion. But that does not seem to be what Tosafot is saying.
Am I missing something here? Is there perhaps a textual emendation that switches the terms in Tosafot around a bit? Or is it possible that Tosafot mixed up some of the opinions?