A selection of the lowest April maxima (<3.0C) from old stations and some synoptic stations.
Not a one to one comparison however as early 20th century values were based on maxima recorded between 09-09 whilst those from synoptic stations are midnight to midnight. So this should be kept in mind.
The Blacksod Point value of -0.6C also seems very skeptical given its location out at sea and not far from Belmullet so I would personally disregard it. The Malin Head 0.6C also seems skeptical. Clones' 1.6C on 15 April 1966 probably the most reliable low max.
I would share your scepticism on that Blacksod low, but at the same time, I'd not rule it out either. A supplement that came with the Met Eireann April 1986 climate summary focused on the snow storm of early April 1917 and according to it, west Mayo was hit particularly bad with some incredible amounts falling in relatively short periods of time. Not sure if this supplement is attached to the online version of that monthly summary, but I am sure I have a hard copy of it here (somewhere!) that I kindly got from Met Eireann themselves years ago. I'll try and root it out and post a snap shot of it here if no online version is available.