Specifically catalyzes the N1-methylation of pseudouridine at position 54 (Psi54) in tRNAs. pseudouridine(54) in tRNA + S-adenosyl-L-methionine = H(+) + N(1)-methylpseudouridine(54) in tRNA + S-adenosyl-L-homocysteine Homodimer. Belongs to the methyltransferase superfamily. TrmY family.