Just behind him on six is Young Munster wide man Derry O'Connor who touched down twice in the second half for the Cookies, who fell behind immediately when McGrath raced in straight from the kick-off.
Place-kicker Willie Staunton did his best to keep the Limerick men in touch at the break (13-9), but with McGrath running amok and Ronan Dillon and flanker Adrian D'Arcy also crossing the whitewash, there was no stopping the north Dubliners as they cruised to their sixth successive league win.
Old Belvedere remain just a point adrift of 'Tarf in the table courtesy of their excellent 27-24 bonus point victory away to St. Mary's College.
Another young player who is catching the eye recently - 'Belvo number 8 Jack Conan - scored the match-winning try in the 72nd minute as the visitors won an absorbing Dublin derby.
'Belvo had built a 15-7 interval advantage with former Leinster out-half Shaun Berne pulling the strings for them.
Josh Eden-Whaitiri's 11th-minute try set them on their way and the injury-enforced introduction of Leo Auva'a into the back row midway through the first half certainly helped their cause.
Winger Mark Corballis had scored 'Belvo's second try before St. Mary's came to life just before the break with young centre Robbie Hudson grabbing a much-needed try for them.
Hudson completed his brace early in the second period, in response to a converted effort from Old Belvedere winger Philip O'Dwyer, as he fastened onto a well-weighted kick through from Leinster prospect Cathal Marsh.
'Belvo were under increasing pressure after losing their hooker Cillian McDonald to the sin-bin and Gavin Dunne added a penalty to the St. Mary's tally.
But Conan drove over in the right corner for the decisive bonus point score, confirming 'Belvo as worthy winners, although St. Mary's deservedly took a losing bonus point with them thanks to replacement Ian O'Neill's consolation try in injury-time.
Meanwhile, Ballynahinch - buoyed by their first win of the league run against Garryowen - pushed Dolphin all the way in Cork before going down to a 25-18 defeat.
The County Down outfit made a positive start at Musgrave Park with Ulster-capped full-back David McIlwaine converting a 30-metre penalty in the eighth minute.
But Dolphin took the lead by the half hour mark as USA international flanker John Quill touched down after an intense period of pressure. Rory Scannell converted for a 7-3 scoreline.
The Cork side added further tries from Munster's Cian Bohane and James Kiernan to take a 19-8 lead into the second half, with loosehead prop Kyle McCall scoring an unconverted try for 'Hinch.
There was little to choose between the teams when winger Ross Adair went over in the corner for 'Hinch's second try, but this time McIlwaine swung over a superb conversion from the touchline.
In the end, Dolphin were just about able to keep their noses in front as Steve Ford's men earned their second win in three league games.
Young out-half Scannell converted two penalties after 60 and 76 minutes - sandwiching a lone strike from McIlwaine - to ensure the home success.
Lansdowne made it four victories in five league outings by blasting clearing of hosts Garryowen in the second half of their meeting at Dooradoyle.
A lone try from Munster's Ronan O'Mahony, eight minutes from time, was all the lowly Light Blues could muster having trailed 18-3 at the break.
Lansdowne, who are noted for their fine back play, got their reward through their forwards on this occasion as prop Jack O'Connell (2) and back rowers Charlie Butterworth and Ron Boucher (2) all dotted down in the 35-10 bonus point win which has moved Mike Ruddock's charges up to fourth in the standings.
At Belfield, former leaders UCD ended their two-match losing streak by overcoming a tough challenge from Cork Constitution on a 23-17 scoreline. Watch the highlights on Irish Rugby TV.
James Thornton and visiting out-half Johnny Holland swapped first half penalties as the sides turned around at 3-all, before the students put their foot down to score 20 unanswered points - Andrew Boyle scored their first try and number 10 Thornton, who finished with a handsome 18-point haul, also touched down.
Cork Con were falling to their third defeat on the trot in Division 1A, but did fight back for a losing bonus point in the closing stages with converted tries from Cathal O'Flaherty and Luke Duffy.
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