This is what I found I believe this to be the correct one but I can appreciate what you have said
Careful - you have two primary records (the 1901 census and a baptism record) that contradict the DOB in the one record you refer to, that's not to say it's 'wrong' but I wouldn't go as far as to say it's "correct" in respect of the Thomas Dempsey referred to, i.e. the one in the 1901 census.
It's possible that the Thomas born in 1878 died in infancy and when another son was born in 1882 they named him Thomas as well. This happened in my Great Grandparents family in Wexford when a 'Patrick' died aged 6 months in 1890 and a subsequent son born in 1897 was then also named 'Patrick'.
Judging by the 1901 census Martin & Bridget initially had children on a regular basis (approx every two years) so if they married in 1877 you would have expected them to have had a child in approx 1878 (Thomas), 1880 (?) 1882 (Thomas) 1884 (Terence) 1886 (Mary) etc.
Below is the top of the page in the baptism register which confirms that 1882 is the date of the baptism I referred to.