Copies of the Dublin Evening Mail (from its inception onwards) are held in the National Library here in Dublin. I'll have a look at the death's notices for the Halpins you've mentioned, but I'm not optimistic about finding much. For example - the Rev. N J Halpin was editor of the paper from about 1837 until (I presume) his death in 1851, so you'd imagine he'd receive a fairly generous obituary. I've looked and found only a couple of lines containing little of value by way of information (I've already posted what I uncovered). That puzzled me, and suggested a possible falling-out with the paper's proprietor. People expressed surprise when it was revealed after his death that the Reverend left little or nothing behind, except debts, perhaps. This suggests to me that the perception people had of N J Halpin was of a man who, at the very least, was comfortably off. I wondered if the reverend's brother - William Henry Halpin - might have had something to do with his declining fortunes. W H Halpin had been declared a bankrupt after investing heavily in the railway mania in the mid to late 1840s. Were there associates and friends of the reverend who had also invested in the railways, at the behest and recommendation of the reverend's brother? If so, they too would have lost heavily and this may account for the silence that seems to surround N J Halpin after his death. The obit in the Dublin Evening Mail was cursory, even begrudging. It's possible that the reverend used his own financial resources to cover his brother's debts, which in turn meant there was nothing left in the kitty to take care of his wife and family after his death. I'm only speculating, of course, but the lack of an expression of real appreciation for Halpin from the paper he served well for so long is a genuine puzzle, and needs to be accounted for.
There's more to consider here - by the time of Halpin's death, the famine had killed upward of a million and had driven many more into exile. Contemporary accounts describe scenes of absolute horror in which emaciated women and children die in ditches, on the sides of country roads, in dark mud huts and at the gates of village cemeteries. Dogs fought over the remains of the dead. If we recall the endeavors of Dr. Charles Halpin, who struggled admirably and desperately to alleviate the effects of the famine before it took such fantastic toll on the poor, we have to wonder what his brother, the reverend, made of it all. He was editor of a newspaper that fiercely supported London's response to the blight - a paper that made a point of attacking Charles Halpin's suggestions for the construction of ventilation pits. We know that at one stage at least, the brothers were close - they had worked together on Lord Farnham's estate cataloging the local flora, and had together discovered an important antiquity. It seems reasonable to me to conclude that the reverend N J Halpin must have regarded the Mail's attack on his brother as regrettable. Could there have been a difference of opinion between N J Halpin and the Dublin Evening Mail over that paper's craven support for the British government's policy response to the famine? If there was, that too could account for the paper's apparent coldness toward Halpin at the time of his death in 1851.
Finally - throughout the 1840s Daniel O'Connell had been struggling to convince the British to repeal the Act of Union and re-establish a form of Home Rule in Ireland. Initially, at least, the Dublin Evening Mail was strident in its opposition to O'Connell, with Halpin leading the way. But a visitor to the country in the mid-1840s (James Grant, Impressions of Ireland and the Irish, London, 1844, p. 276 - see quote below) wrote about his encounters there, and in it he claimed that the Dublin Evening Mail was reconsidering its opposition to repeal. It's possible that this change of heart was prompted by the reverend N J Halpin. As it turned out, the paper held to the party line, and continued to oppose repeal. But it's an incident that may have resulted in Halpin being ostracised by members of the Anglo-Irish community - provided he was in fact behind the paper's flirtation with O'Connell's campaign.
These are just a few things to consider when thinking about the Reverend N J Halpin and his relationship with the Dublin Evening Mail.