They are after the wrong man. Howlin had nothing to do with the decision to change the Irish text to italics, which occurred in 1989, under the watch of Padraig Flynn and was implemented by Regulation 2 of the Road Traffic (Signs) (Amendment) Regulations, 1988:
Subject to the provisions of sub-paragraph (b) place-names on all informative signs provided after 1st January, 1989 shall be shown in italic letters in Irish and in Roman letters in English and where the spelling of a place-name is similar in both languages an Irish inscription in italic letters only need be shown on the sign
The history of this is, the original 1926 Regulations provided for the Irish text to be in Gaelic script (not explicitly stated in the Regulations, but made clear from the illustrations). The 1956 Regulations changed this to the form most older readers will be familiar with, the Irish text being in the Roman alphabet, all caps, but smaller than the English text. There are still quite a few of these signs in situ on rural roads (and even in Dublin city - there will a few in situ at the junction of Infirmary Road and the North Circular last time I checked) if you know where to look.
The "new" signs, based on the Warboys Committee designs from the UK, were introduced in 1977 under Sylvester Barrett and initially used mixed case Transport for the Irish text and all caps Transport for the English text. Why they capitalized the English, I'm not clear since this was obviously not part of the Warboys Committee designs, in which capitals should only be used for Superdestinations (compass points, like The NORTH, The SOUTH, or, oddly, SCOTLAND). The original signs were actually a lot closer to the UK design and part of me thinks the slight redesign they got in 1989 was simply to make them look less British. There were other tweaks made too, the borders (which were originally the same as the UK design) were changed to the weird framed version we use now and there were other design tweaks too.
The 1996 TSM did not make any changes as to how Irish was treated on road signs, although we did abolish superdestinations at that point. The biggest change at that stage was the introduction of the "Guildford Rules" (patching of destinations - before that only road numbers had been patched) although that had already been informally in use, I think the first road I saw it used on was the M4 Leixlip-Kilcock motorway at the terminal junctions and roads directly off it.
The 1996 TSM is very similar to the UK TSM of the same era, even the chapter numbers and headings are the same. They basically tweaked it for Irish use and I doubt many if any graphic designers were involved on the Irish side.