Thanks to a surplus from the previous year, he has managed to retain seven full-time teachers so far, but, with pupil numbers continuing to decline, and funds dropping accordingly, he faces the prospect of making one of his staff redundant.
Thirty miles away Liam McCluskey is principal of another school with precisely the same number of pupils - 136. He too may lose a teacher this year as enrolments fall.
The two heads have much in common. Both are active in the Irish National Teachers' Organisation and their Catholic primary schools serve deprived rural areas. But the financial background is utterly different. Peter McNulty's school is in Northern Ireland, while Liam McCluskey is head of Drumoghill National School across the border in County Donegal.
St Joseph's has seven teachers, Drumoghill has only five and has no classroom assistant, caretaker or secretary, and only a cleaner for one hour per day.
Mr McCluskey has no delegated budget. The meagre government grant, which is handled by the parish treasurer, is swallowed up by heating, lighting, insurance and a few other essentials. He has no print-out of his spending because the school does not own a computer, so forget information technology as a curriculum subject. The Northern Ireland school, by contrast, has 10 computers, nine printers and two CD-Roms.
While the funding system clearly favours Northern Ireland schools, however, teachers' salaries in the Republic are generally higher. A newly qualified teacher earns Pounds 15,075 a year, compared with Pounds 13,350 in the UK. While UK classroom teachers climb up the salary scale faster, earning more after five years, those in the Republic eventually outstrip them, earning up to Pounds 24,950. UK teachers reach a ceiling at Pounds 20,145 and can only be paid above this amount by merit or promotion. Most Uk teachers earn a maximum of Pounds 22,665.
According to the Republic's recent White Paper on Education the small level of expenditure represents a huge improvement. Thirty years ago it spent only Pounds 19 million on primary education compared with Pounds 719m this year; spending per pupil has risen threefold over the same period.
But the document shows that spending remains at the bottom of the Organisation for Economic Co-operation and Development's league of developed countries both in terms of funding per pupil and as a proportion of national wealth. For example, expenditure per primary pupil in 1992 was about US$1000 (Pounds 660) less than in the United Kingdom, including teaching costs.
Primary budgets in Northern Ireland are lower than in the rest of the United Kingdom, but their standard of staffing and other resources is closer to British than to Irish norms.
St Joseph's last year had an allocation of just over Pounds 195,000, of which Pounds 157,500 was teachers' costs, leaving Pounds 37,500 if the carried-over surplus is ignored. This makes Pounds 275 per year for each of the 136 pupils - nearly seven times more than Drumoghill.
In fact, the gap is even wider because the local education and library board pays for external maintenance, swimming, insurance and cover for in-service training and sickness.
Peter McNulty welcomes the flexibility he gets with local management of schools. He has converted part of a cloakroom to provide a badly-needed office and last year bought two extra computers.
"LMS has been a great thing for schools. There has been more money in schools and there are great benefits in being able to allocate the money where you see the greatest needs."
Across in Donegal, Liam McCluskey, who is vice-president of INTO as well as head of Drumoghill, has little money and no flexibility over how to spend it. He was far from impressed when this year's grant went up by only Pounds 2 to Pounds 40 per pupil whereas the secondary allowance rose four times more.
"At secondary level the capitation allowance is Pounds 165," he says. "It is absolutely crazy and shows the under-funding of the primary system. We do not get any grants for equipment We have one copier, television and video but the parish had to contribute the money and parents had to fund-raise.
"Except for the generosity of the parents we would have no equipment whatsoever."
Support staff and computers are not the only things lacking. There is no all-purpose room for art or music and no space or equipment for technology or science. There is no money to take children swimming nor anything like a gymnasium for PE or games, so if the weather is bad they don't exercise.
Despite all the advantages that St Joseph's enjoys, falling rolls mean a declining budget and a higher pupil:teacher ratio.
But, even if the Republic's schools get more money the prospect of funding on both sides of the border being the same - one inevitable goal for supporters of a united Ireland - is remote.
Meanwhile, Drumoghill will remain a spartan place, struggling against the odds to teach a full curriculum, while St Joseph's will at least have the space and facilities to offer a proper educational experience to children.